[Diego1618]

I am asking for a specific *historical* source that says that the Apostolic Church apostatized at the time of Constantine and mixed Christian practices with pagan ones.

How about Sunday and Easter? I believe no one can deny the historical involvement of Constantine here. From the "Codex Justinianus"....unfortunately it takes a while to load.

[Stone Mountain]

Or for that mastter, the tree symbolizing Christmas. Easter has rabbits and eggs symbolizing fertility. there are clearly pagan roots in the ways these holidays are celebrated. Why is that a big deal, I don't know...

[Campion]

Both Easter and Sunday worship were being practiced by AD 100. Constantine may have issued decrees concerning them, but that was merely because the Christian faith was legal for the first time during his reign. (Prior to that it was an underground, persecuted sect.)

He wasn't changing accepted Christian practice, just recognizing it formally as the representive of the Roman state.

Don't you think, if the celebration of Pascha had been unknown and everyone worshipped on Saturday, we'd have some record of complaint or protest when those things were changed by a government which had only recently stopped being a persecutor?