Answer:

We cannot be “certain” of anything in the ancient world.  We cannot be “certain” that a person named Muhammad lived.   We cannot be “certain” that Alexander the Great conquered Persia.   We have to admit that discussing certainty in the past is a bit tricky.  We do not have a movie of Alexander the Great on his campaigns.  We have no actual manuscript written by either Muhammad or his secretariesm, only copies.   Certainty about things in the distant past is difficult to obtain.  What we must ask is what is the most reasonable interpretation of the evidence we have.  The evidence makes the reality of Muhammad or Alexander the Great so great that no rational person denies they existed.

The same can be said for Paul, Jesus, Irenaeus, Origen and Justin.  Given the evidence, we can be 99.99% confident that a person named Irenaeus lived and that he wrote a work titled Against Heresies.  We can be 99.99% certain that Justin known as Martyr lived and that he published at least two apologies.  We can be at least 99.99% sure that a person named Origen lived and that he published massive amounts of Christian theology.

What is slightly less certain is the exact content of the writings of Justin and Irenaeus.   I suggest you look to what the consensus of scholarship says.  The consensus of scholarship says that we have at least two genuine apologies of the actual person Justin Martyr.  The consensus of scholarship is that all or nearly all of what we have as Against Heresies is indeed from Irenaeus, and that Irenaeus wrote MUCH more, some of which we also have, but much of which is lost.  The fact that there might be some material interpolated to the writings of these people does little to change what we know of them or what their basic message and teaching was.   For example, with Irenaeus, we can say for sure (99.99%) that he wrote against a number of real gnostic opponents, and we can name several of them.  We know that he used both the Bible and the apostolic tradition to defend his orthodox belief.  If your Muslim opponents demand 100% proof of a specific line in Against Heresies, they are asking something that it is impossible to give and they are asking something that it would be impossible to give with regard to the Qur’an or the Hadith.  With the Qur’an we can be nearly certain that the majority of it reflects things said by Muhammad.  But it is entirely possible that some parts of it are later interpolations and some of it may not have been from Muhammad at all.  Yet, in order to engage in reasonable discussions with Muslims, I respectfully accept, for the sake of argument, that it is all by Muhammad.  This is how Muslims should treat the Christian writings—yet they do not.  This is on them!!!

We should talk about what is almost certainly true, not what is “sure 100%”.   If you need to have manuscripts of a writer which are within a couple of generations of the original writer, then literally nothing written by Julius Caesar, Ovid, Plato, Aristotle, Tacitus or Herodotus is reliable.  We are much more certain of Justin and of Irenaeus than we are of Aristotle or Julius Caesar, yet these writings are received with virtually zero skepticism as fully genuine.  You can accept Against Heresies, Dialogue against Trypho (Justin) and Justin’s first apology as genuine and useful for interpreting the thoughts of these men.  When I write about these men, I write using terminology as if I am certain they wrote a certain passage because I am nearly certain that they did.  I do not bother to say that such and such quote is only 99.9% certainly from this writer.  This is normal human behavior when talking about writings from the ancient world.

As for specifics, a truly ancient manuscript of Justin’s first Apology was announced fairly recently (2013).  It is from the fourth century—two hundred years after it was written by Justin.  The manuscript is only partial, but it supports the validity of the much later Greek manuscripts we have for Justin’s work.   This is taken from  http://www.bricecjones.com/blog/new-discovery-the-earliest-manuscript-of-justin-martyr-poxy-5129

New Discovery: The Earliest Manuscript of Justin Martyr (P.Oxy. 5129)

10/31/2013 10 Comments

In the latest issue of the Oxyrhynchus Papyri series (LXXVIII, Egyptian Exploration Society, 2012), W. B. Henry offers an edition of P.Oxy. 5129 (Justin Martyr’s First Apology), which is the earliest Greek manuscript of any text of Justin Martyr. According to Henry, “[t]his is the first published ancient copy of a work of Justin Martyr. The text is otherwise known only from the unreliable manuscript A (Parisinus graecus 450, of 1364).” Henry dates the hand to the 4th century CE, citing P.Oxy. 2699 and P.Herm. 5 as comparanda. This is, therefore, an incredible discovery, since P.Oxy. 5129 predates the earliest manuscript of Justin by a millennium! There are a few variants in the fragment (e.g., omission of εντυχειν in 50.12, υμων instead of ημων in 51.4) that make the text important for text-critical study of Justin’s First Apology.

The manuscript is written on parchment in an elegant hand of the “Severe” type. Unfortunately, only six, partial lines have been preserved (3 lines on hair, 3 lines on flesh), and the flesh side is particularly sparse. Henry collates the text with the critical edition of D. Minns and P. Parvis (2009). For interested readers, I reproduce Henry’s transcription of the text of P.Oxy. 5129 below, alongside my own translation (with brackets signifying reconstructions), which is followed by a snapshot of the hair side of the fragment.

I am happy to help.

John Oakes