Celsus (/ˈsɛlsəs/; Hellenistic Greek: Κέλσος, Kélsos; fl. AD 175–177) was a 2nd-century Greek philosopher and opponent of early Christianity. His literary work The True Word (also Account, Doctrine or Discourse; Greek: Hellenistic Greek: Λόγος Ἀληθής) survives exclusively via quotations in Contra Celsum, a refutation written in 248 by Origen of Alexandria. The True Word is the earliest known comprehensive criticism of Christianity and Judaism.

Hanegraaff has argued that The True Word was written shortly after the death of Justin Martyr (who was possibly the first Christian apologist), and was probably a response to his work. Origen stated that Celsus was from the first half of the 2nd century AD, although the majority of modern scholars have come to a general consensus that Celsus probably wrote around AD 170 to 180.

All that is known about Celsus himself 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 Platonic 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 The True Word 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 easy 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 the ancient Greek religion and the religion in ancient Rome, 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, to demonstrate the inconsistency of the Christian position, and he also uses Christian arguments among others to deconstruct the Jewish religion.

Celsus was the author of a work titled The True Word (Logos Alēthēs). The argument was contested by the contemporary Christian community and the book was eventually banned in 448 AD by order of Valentinian III and Theodosius II, along with Porphyry's 15 books attacking the Christians, The Philosophy from Oracles. No complete copies are extant, but it can be reconstructed from Origen's detailed account of it in his eight volume refutation, which quotes Celsus extensively. Origen's work has survived and has thereby preserved Celsus's work.

Celsus seems to have been interested in Ancient Egyptian religion, and he seemed to know of Hellenistic Jewish logos-theology, both of which suggest The True Doctrine was composed in Alexandria. Origen indicates that Celsus was an Epicurean living under the Emperor Hadrian.

Celsus writes that "there is an ancient doctrine [archaios logos] which has existed from the beginning, which has always been maintained by the wisest nations and cities and wise men". He leaves Jews and Moses out of those he cites (Egyptians, Syrians, Indians, Persians, Odrysians, Samothracians, Eleusinians, Hyperboreans, Galactophagoi, Druids, and Getae), and instead blames Moses for the corruption of the ancient religion. "The goatherds and shepherds who followed Moses as their leader were deluded by clumsy deceits into thinking that there was only one God, [and] without any rational cause ... these goatherds and shepherds abandoned the worship of many gods". However, Celsus's harshest criticism was reserved for Christians, who "wall themselves off and break away from the rest of mankind".