In several encyclopedias I have now read something to the effect that "Irenaeus is unique among the late second century writers because unlike them, he was actually born into a Christian family and raised as a Christian." I would like to know whether anyone concurs with this statement, which appears to be based on the Letter to Florinus where he mentions hearing Polycarp in his youth. For the moment I will assume that the authenticity and dates of the people and writings mentioned are in some way valid.
From his writings, at least to me, Irenaeus' thought world is not too dissimilar from that of Tatian and (especially) Justin, both of whom go to great lengths to insist that they were attracted to Christianity from paganism; Tatian explicitly states that it was through the LXX while I think it can be assumed that Justin went through a similar thought process based on his heavy reliance on the prophetic spirit of the OT. He seems to absolutely delight in LXX proof texts that usually do violence to the text, something we see in Irenaeus, especially in his "Demonstration" and also in quite a few of Harvey's fragments (most of them seem spurious but the commentaries on the Book of Numbers sure sound like Irenaeus). These writers, like Irenaeus, also scorn philosophy but indicate familiarity with a wide range of doctrines. I don't know that there is anything specific I could point to as revealing that these perspectives are from a lifelong Christian; rather, an intellectually-minded pagan who accepted the "Word made flesh" variation of Christianity and based it around the intellectual exercise of proofing from the LXX.
If we then take the contemporary examples of Tertullian (paganism), Athenagoras (paganism), Diognetus (almost certainly paganism), Clement of Alexandria (paganism), Melito (Judaism), Aristides (a convert from paganism per Jerome) then Irenaeus seems even more like the odd man out. Would there be any qualitative evidence from his writings that he was a Christian from birth?