Diogenes was a Greek writer that lived during the 3rd century CE; Vitae is his work on the Lives of the Philosophers, and he makes mention of two lost works that make mention of the Druids - an apocryphal book by Aristotle and a book by Sotion of Alexandria, who wrote during the 2nd century BCE.

P. Cornelius Tacitus was one of the greatest Roman historians. His Annals covered the period from 68 CE (Nero's death) until 98 CE and his Histories covered the period from 4 CE until 68 CE (the reigns of Tiberius, Caligula, Claudius, and Nero), although only portions from each work have survived.Pliny the Elder's Natural History was published c. 77 BCE and contains much ritual information that previous sources did not mention. However, during the time of Pliny, the Druids had acquired a stereotypical portrayal as "seers" or "magicians", and for this reason, Pliny's comments concerning the Druids should be used very cautiously. It is from the following quotes of Pliny that most of the antiquarian ideas concerning Druidism originated

M. Annaeus Lucanus (Lucan) lived during the reign of the emperor Nero and his epic poem Pharsalia concerns the civil war between Caesar and Pompey the Great, the poem taking it's name from Pharsalus where Caesar defeated Pompey and gained a short-lived sole-control of Rome.

Diodorus Siculus was a Greek writer that wrote sometime c. 8 BCE. By the time of Diodorus, the Druids were beginning to be viewed only as seers or magicians.

M. Tullius Cicero was a great Roman orator who was a close friend with the Druid Divitiacus, who is also mentioned in Caesar's De Bello Gallico. His work De Divinatione (Concerning Divination) was written c. 44 BCE as a treatise on various methods of divination as practiced by different peoples. C. Julius Caesar was a Roman general and politician whose De Bello Gallico (Commentary on the Gallic War) was written c. 52 BCE. Although Caesar had first-hand knowledge of the Gauls and the Druids, his commentaries were political propaganda so that the Senate of Rome would allow him to continue his exploits against the Gauls, a war that was a vital step to the civil wars with C. Pompeius Magnus (Pompey the Great) a few years later. His commentaries were also written as a sort of guide for later historians writing a history about the Gallic Wars, a straight forward commentary that is free of embellishments.

Caesar, De Bello Gallico, 6.13

Throughout all Gaul there are two orders of those men who are of any rank and dignity: for the commonality is held almost in the condition of slaves, and dares to undertake nothing of itself, and is admitted to no deliberation. The greater part, when they are pressed either by debt, or the large amount of their tributes, or the oppression of the more powerful, give themselves up in vassalage to the nobles, who possess over them the same rights without exception as masters over their slaves. But of these two orders, one is that of the Druids, the other that of the knights. The former are engaged in things sacred, conduct the public and the private sacrifices, and interpret all matters of religion. To these a large number of the young men resort for the purpose of instruction, and they [the Druids] are in great honor among them. For they determine respecting almost all controversies, public and private; and if any crime has been perpetrated, if murder has been committed, if there be any dispute about an inheritance, if any about boundaries, these same persons decide it; they decree rewards and punishments; if any one, either in a private or public capacity, has not submitted to their decision, they interdict him from the sacrifices. This among them is the most heavy punishment. Those who have been thus interdicted are esteemed in the number of the impious and the criminal: all shun them, and avoid their society and conversation, lest they receive some evil from their contact; nor is justice administered to them when seeking it, nor is any dignity bestowed on them. Over all these Druids one presides, who possesses supreme authority among them. Upon his death, if any individual among the rest is pre-eminent in dignity, he succeeds; but, if there are many equal, the election is made by the suffrages of the Druids; sometimes they even contend for the presidency with arms. These assemble at a fixed period of the year in a consecrated place in the territories of the Carnutes, which is reckoned the central region of the whole of Gaul. Hither all, who have disputes, assemble from every part, and submit to their decrees and determinations. This institution is supposed to have been devised in Britain, and to have been brought over from it into Gaul; and now those who desire to gain a more accurate knowledge of that system generally proceed thither for the purpose of studying it.

Caesar, De Bello Gallico, 6.14

The Druids do not go to war, nor pay tribute together with the rest; they have an exemption from military service and a dispensation in all matters. Induced by such great advantages, many embrace this profession of their own accord, and [many] are sent to it by their parents and relations. They are said there to learn by heart a great number of verses; accordingly some remain in the course of training twenty years. Nor do they regard it lawful to commit these to writing, though in almost all other matters, in their public and private transactions, they use Greek characters. That practice they seem to me to have adopted for two reasons; because they neither desire their doctrines to be divulged among the mass of the people, nor those who learn, to devote themselves the less to the efforts of memory, relying on writing; since it generally occurs to most men, that, in their dependence on writing, they relax their diligence in learning thoroughly, and their employment of the memory. They wish to inculcate this as one of their leading tenets, that souls do not become extinct, but pass after death from one body to another, and they think that men by this tenet are in a great degree excited to valor, the fear of death being disregarded. They likewise discuss and impart to the youth many things respecting the stars and their motion, respecting the extent of the world and of our earth, respecting the nature of things, respecting the power and the majesty of the immortal gods.

Caesar, De Bello Gallico, 6.16

The nation of all the Gauls is extremely devoted to superstitious rites; and on that account they who are troubled with unusually severe diseases, and they who are engaged in battles and dangers, either sacrifice men as victims, or vow that they will sacrifice them, and employ the Druids as the performers of those sacrifices; because they think that unless the life of a man be offered for the life of a man, the mind of the immortal gods can not be rendered propitious, and they have sacrifices of that kind ordained for national purposes. Others have figures of vast size, the limbs of which formed of osiers they fill with living men, which being set on fire, the men perish enveloped in the flames. They consider that the oblation of such as have been taken in theft, or in robbery, or any other offense, is more acceptable to the immortal gods; but when a supply of that class is wanting, they have recourse to the oblation of even the innocent

Caesar, De Bello Gallico, 6.17

They worship as their divinity, Mercury in particular, and have many images of him, and regard him as the inventor of all arts, they consider him the guide of their journeys and marches, and believe him to have great influence over the acquisition of gain and mercantile transactions. Next to him they worship Apollo, and Mars, and Jupiter, and Minerva; respecting these deities they have for the most part the same belief as other nations: that Apollo averts diseases, that Minerva imparts the invention of manufactures, that Jupiter possesses the sovereignty of the heavenly powers; that Mars presides over wars. To him, when they have determined to engage in battle, they commonly vow those things which they shall take in war. When they have conquered, they sacrifice whatever captured animals may have survived the conflict, and collect the other things into one place. In many states you may see piles of these things heaped up in their consecrated spots; nor does it often happen that any one, disregarding the sanctity of the case, dares either to secrete in his house things captured, or take away those deposited; and the most severe punishment, with torture, has been established for such a deed.

Caesar, De Bello Gallico, 6.18

All the Gauls assert that they are descended from the god Dis, and say that this tradition has been handed down by the Druids. For that reason they compute the divisions of every season, not by the number of days, but of nights; they keep birthdays and the beginnings of months and years in such an order that the day follows the night. Among the other usages of their life, they differ in this from almost all other nations, that they do not permit their children to approach them openly until they are grown up so as to be able to bear the service of war; and they regard it as indecorous for a son of boyish age to stand in public in the presence of his father.

Caesar, De Bello Gallico, 6.21

The Germans differ much from these usages, for they have neither Druids to preside over sacred offices, nor do they pay great regard to sacrifices.

Cicero, De Divinatione, I, xli, 90

Nor is the practice of divination disregarded even among uncivilized tribes, if indeed there are Druids in Gaul - and there are, for I knew one of them myself, Divitiacus, the Aeduan, your guest and eulogist.
He claimed to have that knowledge of nature which the Greeks call physiologia, and he used to make predictions, sometimes by means of augury and sometimes by means of conjecture

Diodorus Siculus, Histories, V, 28, 6

The Pythagorean doctrine prevails among them (the Gauls), teaching that the souls of men are immortal and live again for a fixed number of years inhabited in another body.

Diodorus Siculus, Histories, V, 31, 2-5

And there among them (the Gauls) composers of verses whom they call Bards; these singing to instruments similar to a lyre, applaud some, while they vituperate others.
They have philosophers and theologians who are held in much honor and are called Druids; they have sooth-sayers too of great renown who tell the future by watching the flight of birds and by observation of the entrails of victims; and every one waits upon their word. When they attempt divination upon important matters they practice a strange and incredible custom, for they kill a man by a knife-stab in the region above the midriff, and after his fall they foretell the future by the convulsions of his limbs and the pouring of his blood, a form of divination in which they have full confidence, as it is of old tradition. It is a custom of the Gauls that no one performs a sacrifice without the assistance of a philosopher, for they say that offerings to the gods ought only to be made through the mediation of these men, who are learned in the divine nature and, so to speak, familiar with it, and it is through their agency that the blessings of the gods should properly be sought. It is not only in times of peace, but in war also, that these seers have authority, and the incantations of the bards have effect on friends and foes alike. Often when the combatants are ranged face to face, and swords are drawn and spears bristling, these come between the armies and stay the battle, just as wild beasts are sometimes held spellbound. Thus even among the most savage barbarians anger yields to wisdom, and Mars is shamed before the Muses.

Strabo, Geographica, IV, 4, c. 197, 4

Among all the Gallic peoples, generally speaking, there are three sets of men who are held in exceptional honor: the Bards, the Vates, and the Druids. The Bards are singers and poets; the Vates, diviners and natural philosophers; while the Druids, in addition to natural philosophy, study also moral philosophy. The Druids are considered the most just of men, and on this account they are entrusted with the decision, not only of the private disputes, but of the public disputes as well; so that, in former times, they even arbitrated cases of war and made the opponents stop when they were about to line up for battle, and the murder cases in particular, had been turned over to them for decision. Further, when there is a big yield (of criminals for sacrifice) from these cases, there is forthcoming a big yield from the land too, as they think. However, not only the Druids, but others as well, say that men's souls, and also the universe, are indestructable, although both fire and water will at some time or other prevail over them.

Strabo, Geographica, IV, 4, c.198, 5

But the Romans put a stop to these customs, as well as to all those connected with the sacrifices and divinations that are opposed to our usages. They used to strike a human being, whom they had devoted to death, in the back with a sabre, and then divine from this death-struggle. But they would not sacrifice without the Druids. We are told of still other kinds of human sacrifices; for example, they would shoot victims to death with arrows, or impale them in the temples, or having devised a colossus of straw and wood, throw into the colossus cattle and wild animals of all sorts and human beings, and then make a burnt offering of the whole thing.

Lucan, Pharsalia, I, 450-458

And Druids, you laid down your weapons and returned to your barbaric rites and weird manner of ceremonial. To you alone is granted total knowledge of the gods and heaven's powers -- or total ignorance. Inhabiting deep groves in remote woods, you teach that ghosts do not head for Erebus' silent home or for the colorless realm of Dis below, but that the self-same spirit rules the limbs in another sphere.

Pliny, Natural History, XVI, 249

Here we must mention the awe felt for this plant by the Gauls. The Druids - for so their magicians are called - held nothing more sacred than the mistletoe and the tree that bears it, always supposing that tree to be the oak. But they choose groves formed of oaks for the sake of the trees alone, and they never perform any of their rites except in the presence of a branch of it; so that it seems probable that the priests themselves may derive their name from the Greek word for that tree. In fact, they think that everything that grows on it has been sent from heaven and is a proof that the tree was chosen by the god himself. The mistletoe, however, is found but rarely upon the oak; and when found, is gathered with due religious ceremony, if possible on the sixth day of the moon (for it is by the moon that they measure
their months and years, and also their ages of thirty years). They choose this day because the moon, though not yet in the middle of her course, has already considerable influence. They call the mistletoe by a name meaning, in their language, the all-healing. Having made preparation for sacrifice and a banquet beneath the trees, they bring thither two white bulls, whose horns are bound then for the first time. Clad in a white robe, the priest ascends the tree and cuts the mistletoe with a golden sickle, and it is received by others in a white cloak. They they kill the victims, praying that the god will render this gift of his propitious to those to whom he has granted it. They believe that the mistletoe, taken in drink, imparts fecundity to barren animals, and that it is an antidote for all poisons. Such are the religious feelings that are entertained towards trifling things by many peoples.

Pliny, Natural History, XXIV, 103-104

Similar to savin is the plant called selago. It is gathered without using iron and by passing the right hand through the left sleeve of the tunic, as though in the act of committing a theft. The clothing must be white, the feet washed and bare, and an offering of wine and bread made before the gathering. The Druids of Gaul say that the plant should be carried as a charm against every kind of evil, and that the smoke of it is good for diseases of the eyes.
The Druids, also, use a certain marsh-plant that they call samolus, this must be gathered with the left hand, when fasting, and is a charm against the diseases of cattle. But the gatherer must not look behind him, nor lay the plant anywhere except in the drinking-troughs.

Pliny, Natural History, XXIX, 52

There is also another kind of egg, of much renown in the Gallic provinces, but ignored by the Greeks. In the summer, numberless snakes entwine themselves into a ball, held together by a secretion from their bodies and by their spittle. This is called anguinum. The Druids say that hissing serpents throw this up into the air, and that it must be caught in cloak, and not allowed to touch the ground; and that one must instantly take to flight on horseback, as the serpents will pursue until some stream cuts them off. It may be tested, they say, by seeing if it floats against the current of a river, even though it be set in gold. But as it is the way of magicians to cast a cunning veil about their frauds, they pretend that these eggs can only be taken on a certain day of the moon, as though it rested with mankind to make the moon and the serpents accord as to the moment of the operation. I myself, however, have seen of these eggs; it was round, and about as large as a smallish apple; the shell was cartalaginous, and pocked like the arms of a polypus. The Druids esteem it highly. It is said to ensure success in law-suits and a favorable reception with princes; but this is false, because a man of the Vocontii, who was also a Roman knight, kept one of these eggs in his bosom during a trial, and was put to death by the Emperor Claudius, as far as I can see, for that reason alone.

Pliny, Natural History, XXX, 13

It [magic] flourished in the Gallic provinces, too, even down to a period within our memory; for it was in the time of theEmperor Tiberius that a decree was issued against the Druids and the whole tribe of diviners and physicians. But why mention all this about a practice that has even crossed the ocean and penetrated to the utmost parts of the earth? At the present day, Britannia is still fascinated by magic, and performs its rites with so much ceremony that it almost seems as though it was she who had imparted the cult to the Persians. To such a degree do peoples throughout the whole world, although unlike and quite unknown to one another, agree upon this one point.
Therefore we can not too highly appreciate our debt to the Romans for having put an end to this monstrous cult, whereby to murder a man was an act of the greatest devoutness, and to eat his flesh most benefial

Tacitus, Annals, XIV, 30

The enemy lined the shore in a dense armed mass. Among them were black-robed women with dishevelled hair like Furies, brandishing torches. Close by stood Druids, raising their hands to heaven and screaming dreadful curses. This weird spectacle awed the Roman soldiers into a sort of paralysis. They stood still - and presented themselves as a target. But then they urged each other (and were urged by their general) not to fear a horde of fanatical women. Onward pressed their standards and they bore down their opponents, enveloping them in the flames of their own torches. Suetonius garrisoned the conquered island. The groves devoted to Mona's barbarous superstitions he demolished. For it was their religion to drench their altars in the blood of prisoners and consult their gods by means of human entrails.

Tacitus, Histories, IV, 54

The Gauls, they remembered, had captured the city in former days, but, as the abode of Jupiter was uninjured, the Empire had survived; whereas now the Druids declared, with the prophetic utterances of an idle superstition, that this fatal conflagration was a sign of the anger of heaven.

Diogenes Laertius, Vitae, Intro. I

Some say that the study of philosophy was of barbarian origin. For the Persians had their Magi, the Babylonians or the Assyrians the Chaldeans, the Indians their Gymnosophists, while the Kelts and the Galatae had seers called Druids and Semnotheoi, or so Aristotle says in the "Magic," and Sotion in the twenty-third book of his "Succession of Philosophers."

Diogenes Laertius, Vitae, Intro. 5

Those who think that philosophy is an invention of the barbarians explain the systems prevailing among each people. They say that the Gymnosophists and Druids make their pronouncements by means of riddles and dark sayings, teaching that the gods must be worshipped, and no evil done, and manly behavior maintained