I really don’t see what any of this has to do with the topic at hand. I didn’t use the word “fantasy”. You said that it is “common knowledge” that James was not really Jesus’ brother, but while this may be a common belief in some traditions, it is not knowledge, and my saying that is not a denigration of that belief or of “faith” in general, whatever you mean by that. It’s a simple historical fact that the claim that James was not really Jesus’ brother is not found in the New Testament, and was a later development, and it was based not on historical evidence but on the doctrine of Mary’s perpetual virginity (a doctrine which itself has no biblical basis either). Perhaps my use of the word “invented” was a bit brusque, so I apologise for that; what I meant by it was to emphasis the point I’ve just made, that it was later and not based on any historically reliable sources.
That doesn’t mean it isn’t true, and I didn’t say that we can be certain it isn’t true. I merely said that there is no good reason, from a historical point of view, to think that it is true. So while Catholics are perfectly entitled to say that they believe it to be true on the basis of other articles of faith, they’re not entitled to assert that it is generally known to be true, because it isn’t - even if by chance it happens to be true - because there is no historical evidence to support it. True belief is not the same thing as knowledge.
To say this isn’t to diminish “faith” or to presuppose a position of atheism. Nothing I’ve said here assumes atheism, or the falsity of Christianity, or anything of the kind. But this thread is in the History subforum and it is about historical questions about Jesus’ birth, and questions of that sort can only be settled by historical evidence, not by religious belief. Because if you allow religious faith to determine questions of historical fact then you lose all objectivity, since different religious traditions say different things. (I’ve heard devout Protestants denounce the doctrine of Mary’s perpetual virginity and its associated beliefs as contrary to the Christian faith! So if we’re going to allow religious faith to weigh in on such things, how do we decide whom to believe - the Catholics or the Protestants? Or the Jews, or the Muslims, or the Valentinian Gnostics?)
FWIW I don’t think it’s at all plausible to suppose that atheism requires as much “faith” as religious belief - at least not on any plausible definition of those words - and I’d be happy to debate that issue elsewhere, but it’s really off-topic as far as this thread is concerned.
(I hope this doesn’t come across as too strident. I don’t intend to be rude about anyone’s beliefs. But like I say, religious faith or the lack of it is not, in itself, a substitute for evidence when it comes to historical questions.)