Against Celsus (Ancient Greek: Κατὰ Κέλσου, Kata Kelsou; Latin: Contra Celsum), preserved entirely in Greek, is a major apologetics work by the Church Father Origen of Alexandria, written in around 248 AD, countering the writings of Celsus, a pagan philosopher and controversialist who had written a scathing attack on Christianity in his treatise The True Word (Λόγος Ἀληθής, Logos Alēthēs). Among a variety of other charges, Celsus had denounced many Christian doctrines as irrational and criticized Christians themselves as uneducated, deluded, unpatriotic, close-minded towards reason, and too accepting of sinners. He had accused Jesus of performing his miracles using black magic rather than actual divine powers and of plagiarizing his teachings from Plato. Celsus had warned that Christianity itself was drawing people away from traditional religion and claimed that its growth would lead to a collapse of traditional, conservative values.

Origen wrote Contra Celsum at the request of his patron, a wealthy Christian named Ambrose, who insisted that a Christian needed to write a response to Celsus. In the treatise itself, which was aimed at an audience of people who were interested in Christianity but had not yet made the decision to convert, Origen responds to Celsus's arguments point-by-point from the perspective of a Platonic philosopher. After having questioned Celsus's credibility, Origen goes on to respond to Celsus's criticism with regard to the role of faith in Christianity, the identity of Jesus Christ, the allegorical interpretation of the Bible, and the relation between Christianity and traditional Greek religion.

Modern scholars note that Origen and Celsus actually agree on many points of doctrine, with both authors emphatically rejecting conventional notions of anthropomorphic deities, idolatry, and religious literalism. Contra Celsum is considered to be one of the most important works of early Christian apologetics; the church historian Eusebius lauded it as an adequate rebuttal to all criticisms the church would ever face, and it continued to be cited throughout late antiquity.

The pagan philosopher Celsus had written a polemic entitled The True Word (Greek: Λόγος Ἀληθής, Logos Alēthēs), in which he had advanced numerous arguments against Christianity. Celsus refers to the Neopythagorean philosopher Numenius of Apamea, who lived during the late second century AD, on four occasions. This indicates that Celsus must have lived no earlier than the late second century. Many scholars have dated The True Word specifically to the reign of the Roman Emperor Marcus Aurelius (121–180 AD), due to Celsus's argument in Book VIII in which he promotes the ideas of duty to the state in both worship and in war, which are similar to ideas described by Marcus Aurelius in his Meditations. Robert Louis Wilken dates it to around 170 AD.

All that is known about Celsus personally is what comes from the surviving text of his book and from what Origen says about him. Although Origen initially refers to Celsus as an "Epicurean", his arguments reflect ideas of the Platonizing tradition, rather than Epicureanism. Origen attributes this to Celsus's inconsistency, but modern historians see it instead as evidence that Celsus was not an Epicurean at all. Joseph Wilson Trigg states that Origen probably confused Celsus, the author of The True Word, with a different Celsus, who was an Epicurean philosopher and a friend of the Syrian satirist Lucian. Celsus the Epicurean must have lived around the same time as the author of Contra Celsum and he is mentioned by Lucian in his treatise On Magic. Both Celsus the friend of Lucian and Celsus the author of The True Word evidently shared a passionate zeal against superstitio, making it even easier to see how Origen could have concluded that they were the same person.

Stephen Thomas states that Celsus may not have been a Platonist per se, but that he was clearly familiar with Plato. Celsus's actual philosophy appears to be a blend of elements derived from Platonism, Aristotelianism, Pythagoreanism, and Stoicism. Wilken likewise concludes that Celsus was a philosophical eclectic, whose views reflect a variety of ideas popular to a number of different schools. Wilken classifies Celsus as "a conservative intellectual", noting that "he supports traditional values and defends accepted beliefs". Theologian Robert M. Grant notes that Origen and Celsus actually agree on many points: "Both are opposed to anthropomorphism, to idolatry, and to any crudely literal theology." Celsus also writes as a loyal citizen of the Roman Empire and a devoted believer in Greco-Roman paganism, distrustful of Christianity as new and foreign.

Thomas remarks that Celsus "is no genius as a philosopher". Nonetheless, most scholars, including Thomas, agree that Origen's quotations from The True Word reveal that the work was well-researched. Celsus demonstrates extensive knowledge of both the Old and New Testaments and of both Jewish and Christian history. Celsus was also closely familiar with the literary features of ancient polemics. Celsus seems to have read at least one work by one of the second-century Christian apologists, possibly Justin Martyr or Aristides of Athens. From this reading, Celsus seems to have known which kinds of arguments Christians would be most vulnerable to. He also mentions the Ophites and Simonians, two Gnostic sects that had almost completely vanished by Origen's time. One of Celsus's main sources for Books I–II of The True Word was an earlier anti-Christian polemic written by an unknown Jewish author, whom Origen refers to as the "Jew of Celsus". This Jewish source also provides well-researched criticism of Christianity and, although Celsus was also hostile to Judaism, he occasionally relies on this Jewish author's arguments.

Contra Celsum was probably written in around 248 while Origen was living in Caesarea. According to the church historian Eusebius (c. 260 – c. 340 AD), Origen was over sixty years old when he began writing it. He was first introduced to Celsus's True Word by his friend and patron, a wealthy Christian named Ambrose. It is unclear how well known the book was at the time; Origen had previously never heard of it and Ambrose is the first Christian known to have read it. Joseph Wilson Trigg suggests that Ambrose may have been first exposed to the book through encounters with influential pagan intellectuals, who may have been turning to it to explain the ongoing decline of the Roman Empire as the ab urbe condita calendar approached the end of its first millennium. In any case, Ambrose considered the book an imminent threat to the continued growth of the Christian faith and believed that Origen needed to write a rebuttal to it.