I am a current church attending Christian, if that helps? You have accused me of lying and sockpuppeting earlier, but hey, bygones...
what's your position on the reliability of scripture as primary evidence?
It’s unclear what you’re asking here: which ‘scripture’, evidence for what? There are books of the Bible that are good primary evidence for the developing beliefs and practice for the early church, because that’s what they are, they’re letters from someone telling someones else to among other things stop doing so much of X and start doing more of Y. The Gospels aren’t contemporary journalism, but they too are fairly good evidence for what the early church believed and how different strains of thought were changing and developing. So you can see those different strains in the gospels, based on what the compilers/authors felt was important and what they valued.
Any thoughts on the alternative 'gospels' and the early churches decision to pretend they don't exist
The point at which the Church settled for good (mostly...) on which books go in the canon and which don’t happened several hundred years after the ‘early church’ I’m talking about above, for clarity. Also: they didn’t “pretend they don’t exist”, how can you have big discussions about which writings count and which don’t unless you’re acknowledging they’re all there?
What you’re asking (I’m assuming) is actually “when and why did some of these writings end up in the Bible and some not”, which is a much broader and more interesting question.
Short version of ‘when’: by about AD 200 the Church had roughly developed a core canon, of the four gospels we have as canonical today plus some of Paul’s letters and some of the other books in today’s NT. But the Church was not very centralised at this point, and each region had its own list of canonical writings in addition to that. It wasn’t until much later that the Church developed a single “Bible” as we would recognise it today.
Short version of ‘why’: a combination of religious, political and historical reasoning. So some books got left out because the Church was centralising where it stood theologically and politically, and ditching texts produced according to schools of thought it decided fell outside that (Marcionism, Gnosticism). Or because they’re a collection of sayings rather than a narrative document and that’s not where the Church wanted to go at the time. Or because they’d been lost along the way - it’s likely a written text existed that Matthew and Luke were based on, but we’ve lost that now. We know of a lot of “Gospels” that just don’t exist any more.
If you’re asking about why the Infancy Gospel of Thomas specifically didn’t make it in: probably because it was thought to have been written a fair bit later than the canonical four, and by a Greek author with little knowledge of Judaism, and because it’s very Gnostic and Gnosticism was out by the time the Church was settling on a canon. (It did hang around for a fair while, though, we know some early churches were including that gospel in IIRC the early 2nd century CE.)
If you’re asking “why is the Infancy Gospel of Thomas written the way it is” - search me, Gnosticism was weird. I would suggest that a) the Greek author was more heavily influenced by the Greek pantheon than Jewish tradition, so was talking about the infant Jesus as he would have talked about an infant Greek god or demigod; and b) Gnosticism really liked weird, puzzling material that can only be understood via secret knowledge only available to the initiate.
(I appreciate that this is not what you’re looking for - you want a very simplistic bunch of theists with zero knowledge or nuance saying things like “we know the Bible is true because the Bible says so!”. Apologies for things being more complicated than you were aiming for, I suppose. But complexity is interesting! I really recommend getting into it some time!)