According to the Christian father Origen, Celsus was a 2nd-century Greek philosopher and opponent of Early Christianity. He is known for his literary work, The True Word, which survives exclusively in Origen's quotations from it in Contra Celsum. This work, c. 177 is the earliest known comprehensive attack on Christianity.
This work was lost, but we have Origen's account of it in his writings. It was during the reign of Philip the Arab that Origen received this work for rebuttal. Origen's refutation of The True Word contained its text, interwoven with Origen's replies. Origen's work has survived and thereby preserved Celsus' work with it.
Celsus seems to have been interested in Ancient Egyptian religion, and he seemed to know of Jewish logos-theology, both of which suggest The True Word was composed in Alexandria. Celsus wrote at a time when Christianity was being persecuted and when there seems to have been more than one emperor.
As an anti-Christian Greek philosopher, Celsus mounted an attack on Christianity. Celsus wrote that some Jews said Jesus' father was a Roman soldier named Pantera. The views of Celsus drew responses from Origen who considered it a fabricated story.