Celsus Read Irenaeus's Against Heresies

Indeed let's stop for a moment and wonder why Celsus thinks he has 'all knowledge' when it comes to Christianity. One could argue - as some have - that the pagan was just a blowhard boasting beyond measure about his familiarity with Christianity.  Yet that is a dangerous assumption and is contradicted explicitly on several places by Origen.  If all the evidence is taken into account Origen's point against Celsus here is more nuanced.  He argues that Celsus has read many of the books of the Christians but no one has 'all knowledge' about anything.

Yet I think we can go even further here.  We may start to wonder if Celsus has come across Irenaeus or some other treatise 'against all [Christian] schools' (κατὰ πασῶν αἱρέσεων) in the library and based his claims on his intimacy with that text.  There is a strange consistency in Origen's ridicule of Celsus' 'knowing the whole account' ἐπαγγελλόμενος εἰδέναι τὰ τοῦ λόγου πάντα (1.54) of the gospel, his being a "claimant to universal knowledge" (ὁ πάντ' ἐπαγγελλόμενος εἰδέναι 2.1), "being acquainted with everything, inscribed upon his book the words, A True Discourse" (πάντ' εἰδέναι ἀληθῆ λόγον ἐπέγραψεν αὐτοῦ τὸ βιβλίον 1.40).

There is an uncanny similarity in the bombast of Irenaeus throughout his exposition against all the sects. He declares in Book One that "it is a complex and multiform task to detect and convict all the schools, and since our design is to reply to them all according to their special characters (Adv Haer 1.22.2). Irenaeus goes on to say "I have also made a collection of their writings ... It was necessary clearly to prove, that, as their very opinions and regulations exhibit them ... They have now all been exposed." (ibid 1.31.2,3)  In other words, Celsus assumes he has 'all knowledge' of the schools of Christianity because this is what Irenaeus promises throughout his five volume work. This is reinforced at the beginning of each of the books that follow.

In Book Three - "you hast indeed enjoined upon me, my very dear friend ... to exhibit both their doctrines and successions, and to set forth arguments against them all ... I have sent unto thee books, of which the first comprises the opinions of all these men, and exhibits their customs, and the character of their behaviour. Call to mind then, the things which I have stated in the two preceding books, and, taking these in connection with them, thou shalt have from me a very copious refutation of all the heretics." Book Four assures readers like Celsus "by transmitting to thee, my very dear friend ... you may obtain from me the means of confuting all the schools everywhere, and not permit them, beaten back at all points, to launch out further into the deep of error."  And again at the beginning of the last book "in the four preceding books, my very dear friend, which I put forth to thee, all the schools have been exposed ... [I have accomplished this by adducing] something from the doctrine peculiar to each of these men, which they have left in their writings, as well as by using arguments of a more general nature, and applicable to them all."

The fact that it is generally acknowledged that Irenaeus and Celsus were contemporaries with one another it is extremely unlikely that Celsus's bravado was simply a product of an unstable mind.  Celsus thought he knew everything about Christianity because he was reading Irenaeus's refutation of all Christian schools in a public library likely in Rome.