Celsus was a Greek philosopher of the 2nd century CE living within the Roman Empire. He is renowned for authoring a major critique of early Christianity. His precise dates, birthplace, and personal life remain unrecorded. He is generally identified as a Platonist philosopher, though one ancient source also referred to him as an Epicurean. He was clearly well-educated, drawing upon classical Greek philosophy, literature, and Jewish texts to formulate his arguments.

Celsus wrote one known work, The True Doctrine, which constitutes a comprehensive attack on Christian beliefs and practices. The original text is lost, but its content is preserved through extensive quotations and summaries within a later refutation written by the Christian theologian Origen of Alexandria. In his work, Celsus argued that Christianity was an irrational superstition that undermined traditional Roman religion and social order. He criticized core Christian doctrines, mocked Christian practices, and asserted that Christian teachings were derivative of older Greek and Jewish ideas.

Celsus holds significant historical importance as the author of the earliest known sophisticated pagan critique of Christianity. According to modern scholars, his reconstructed text provides a crucial window into the intellectual and religious debates of the 2nd century, revealing how educated pagans perceived the growing Christian movement. His arguments represent a key phase in the conflict between traditional Greco-Roman thought and early Christianity.