Celsus in Harpers Dictionary of Classical Antiquities (1898)
A Platonic, or perhaps Epicurean, philosopher who lived about A.D. 180. His name is famous as that of one of the bitterest enemies of Christianity. From a motive of curiosity, or, perhaps, in order to be better able to combat the new religion, Celsus caused himself to be initiated into the mysteries of Christianity, and to be received into that secret society which St. Clement of Rome is supposed to have founded. It appears, however, that the sincerity of the neophyte was distrusted, and that he was refused admittance into the higher ceremonies. The discontent to which this gave rise in the breast of Celsus inflamed his resentment against the Christians, and he wrote a work against them, entitled Ἀληθὴς Λόγος, "A true discourse," in which he employed all the resources of his intellect and eloquence to paint Christianity as a ridiculous and contemptible system, and its followers as a sect dangerous to the well-being of the State. There is no falsehood to which he has not recourse in order to represent in an untrue light the Christian scheme of morals, to parody and falsify the text of the Old and New Testaments, and to calumniate the character of Jesus Christ and his disciples. He styles Christianity a doctrine tending to pervert and corrupt the human race, and exhorts the government to extirpate the sect if it wishes to save the Empire. The discourse itself is lost; but Origen, who refuted it, in a work divided into eight books, has given us so complete an extract from it that by the aid of this we can follow all the principal reasoning of the author. Celsus wrote also a work against magicians and sorcerers (Κατὰ Μάγων), which is cited by Origen and Lucian. The latter, who was his friend, addressed to him his memoir on Alexander, the false prophet, in which he extols the wisdom of Celsus, his love for truth, and his amiable manners. See Keim, Celsus' wahres Wort (1873); Aubé, La Polémique Païenne (1878); and Pélagaud, Étude sur Celse (1878).
Celsus in Wikipedia
Celsus (Greek: Κέλσος) was a 2nd century Greek philosopher and opponent of Christianity. He is known to us entirely because his literary work, The True Word (Account, Doctrine or Discourse) (Λόγος Ἀληθής), was largely reproduced in excerpts by Origen in his counter-polemic Contra Celsum. The work is the earliest extant anti-Christian polemic.
The author and his work
Celsus was the author of an anti-Christian work titled "The true discourse". The work is lost, but much of it is preserved word-for-word in the pages of a refutation by Origen.
Celsus and his work are unknown other than from the refutation of his work by Origen. Unfortunately Origen knew little about him. He knew that he "has already been dead for a long time" and had heard that there were two philosophers of that name, both Epicureans, one living in the time of Nero, the other in Hadrian's time or later. He treats Celsus as an Epicurean, therefore, although with some hesitation.
Eusebius of Caesarea also mentions the work in his Contra Hieroclem but it is unclear whether he ever saw it, or simply knew Origen's refutation.
Celsus and Lucian
An epicurean philosopher named Celsus is mentioned by Lucian, who dedicates Alexander the false prophet to him. Lucian tells us that his friend wrote a book against magic. But Origen notes that the Celsus who wrote "The True Word" believes in magic, and so queries whether he is the same man who wrote books against magic. The author of The True Word also shows himself to follow Plato and perhaps Philo, rather than Epicurus.. Scholars do not agree on whether the two men are the same, but the consensus is against it.
Date of composition
Celsus wrote at a time when Christianity was being actively persecuted  and when there seems to have been more than one emperor . This would point to either Marcus Aurelius with Lucius Verus (161-9) or Marcus Aurelius with Commodus (177-80). Henry Chadwick reviewed the evidence and preferred the latter period..
Background of Celsus
Celsus' place of origin is very uncertain. He is very well informed about gnostic groups, which were flourishing in Rome in the second century, as well as in Alexandria. He is interested in Egyptian religion, and he seems to know of Jewish logos-theology, both of which suggest Alexandria.
Transmission of the work
During the reign of Philip the Arab (AD 244-9), when Origen was over 60 years old (after 245), the work was sent to the Christian writer Origen for refutation. Origen's refutation Contra Celsum contained the text of Celsus, interwoven with Origen's replies. Origen's work has survived and thereby preserved Celsus with it.
Political environment in which Origen wrote
In 248, although the Church was under no widespread persecution, owing to the inertia or implicit toleration of the emperor Philip the Arab, the atmosphere was full of conflict. Rome was celebrating the 1000th anniversary of its founding, and imperial aspirations and ideas were naturally prominent. The state and the worship of the Caesar, however, were contrasted by Origen with the Christian ideal of a rule and a citizenship beyond this world, to which a thousand years were but as a day. Pride in his faith was blended with a natural anxiety stemming from Celsus' attacks on Christianity, and it was at this point that Origen brought to light again a book written in the days of Marcus Aurelius. Sometimes quoting, sometimes paraphrasing, sometimes merely referring, Origen reproduces and replies to Celsus' arguments. His work shows many signs of haste, but he more than compensates for this by the way in which he thus preserves a singularly interesting memorial of the 2nd century.
Nature of Celsus' attacks
Celsus opens the way for his own attack by restating the arguments leveled at the Christians by the Jews. They are: Jesus was born in adultery and nurtured on the wisdom of Egypt. His assertion of divine dignity is disproved by his poverty and his miserable end. Christians have no standing in the Hebrew Bible prophecies and their talk of a resurrection that was only revealed to some of their own adherents is foolishness. Celsus indeed says that the Jews are almost as ridiculous as the foes they attack; the latter said the savior from Heaven had come, the former still looked for his coming. However, the Jews have the advantage of being an ancient nation with an ancient faith.
To Celsus, it was much more reasonable to believe that each part of the world has its own special deity; prophets and supernatural messengers had appeared in more places than one. Besides being bad philosophy based on fictitious history, Christianity is not respectable. Celsus does not indeed repeat the Thyestean charges (related to parricide, i.e. Christianity overthrowing Judaism) so frequently brought against Christians, but he says the Christian teachers who are mainly weavers and cobblers have no power over men of education.
An interesting feature of Celsus' writing is that he refers to Jesus' father by name as Panthera. It is taken by Celsus as given that Jesus was the illegitimate son of a Roman soldier of this name. There is a tomb of a Roman soldier named Tiberius Iulius Abdes Pantera, which was found in Bad Kreuznach, Germany; some scholars identify the historical Pantera with this Pantera.
Subjects of Celsus' writings
Celsus shows himself familiar with the story of Jewish origins. Any pagan who wished to intimately understand and criticize Christianity had to begin by learning from the Jews, and this accounts for the opening chapters of his argument. He has a good knowledge of Genesis and of the Book of Enoch (v. 52), but does not make much use of the Prophets or the Psalter. Regarding the books with which he was familiar his position is similar to that reflected in the contemporary Acts of the Martyrs of Scili. He speaks of a Christian collection of writings, and knew some parts of the synoptic gospels, but was influenced less by the Gospel of John. There is more evidence of Pauline ideas than of Pauline letters.
The gnostic sects and their writings were well known to Celsus (viii.15 and vi.25), and so was the work of Marcion. There are indications, too, of an acquaintance with Justin Martyr and the Sibylline literature (vii. 53, cp. v.61). He is perfectly aware of the internal differences among Christians, and he is familiar with the various stages of development in the history of their religion. These are cleverly employed in order to heighten the impression of its instability. He plays off the various sects, the primitive age against the present, Christ against the apostles, the various revisions of the Bible against the trustworthiness of the text and so forth, though he admits that everything was not really so bad at first as it is at present.
Influence of Celsus
The True Word had very little influence either on the mutual relations of Christianity and the Roman Empire, or on classical literature. Echoes of it are found in Tertullian and in Minucius Felix, and then it lay forgotten until Origen gave it new life. A good deal of the neo‑Platonic polemic naturally went back to Celsus, and both the ideas and phrases of The True Word are found in Porphyry and Julian, though the closing of the Christian Bible canon in the meantime somewhat changed the method of attack for these writers.
Of more importance than these matters is the light which the book sheds on the strength of Christianity about the year 180. He saw the Christianity of his life time to be simply a number of warring sects (mostly Gnostic), and so seeing only a mark of weakness. Yet there is all through an undercurrent which runs against his surface verdicts, and here and there comes to expression.
Most suggestive, however, is his closing appeal to the Christians: "Come", he says, "don't hold aloof from the common regime. Take your place by the emperor's side. Don't claim for yourselves another empire, or any special position. It is an overture for peace. If all were to follow your example and abstain from politics, the affairs of the world would fall into the hands of wild and lawless barbarians" (viii.68).
Conceding that Christians are not without success in business (infructuosi in negotiis), he wants them to be good citizens, to retain their own belief but conform to the state religion. It is an earnest and striking appeal on behalf of the Empire, and shows the terms offered to the Christian sects, as well as the importance of the various sects at the time. Numerically, Christians formed perhaps a tenth of the population, i.e. in Alexandria there would be 50,000-60,000. It is unlikely their influence was greater than what the physical evidence reveals throughout AD 100-400.