Much of this sounds like speculation. How exactly, do you know what religious beliefs the slaves in Rome had, and why they chose not to talk about particular aspects of their beliefs?
From historians and accounts of the era -- you do know that there are several volumes of scholarly studies on the matter, right? Check out especially Rodney Stark and Jennifer Glancy (but you can also read accounts from that very era too -- up to the early Byzantine era, including from founding figures of the church).