Did Christianity Absorb any Ideas from the Ancient Egyptian Mythology?

I KNEW IT! It always sounded Akhenatenish to me. Some of the other Psalms have an Egyptian feel to me, but that one stuck out like a sore thumb while being theologically okay.

It was one of several possible dates.

Right. It was only offered as one of several possibilities, and as far as we can tell, not until late into the 4th century.

So the birthday of the invincible sun was NOT set on December 25th, and it did not predate Jesus Christ. On top of that, the church only offered this date as one of multiple possibilities for the birth of Christ, and not until fairly late into its existence. In light of all those inconsistencies, I think the claim that Christ’s birthdate was borrowed from that of the Invincible Sun is a real stretch.

Which kinda makes the whole “Mithra came out of solid rock rather than being born of a virgin” business more than just a minor inconsistency.