Is There a Movement Toward Monotheism in the Roman Empire?

Bart erman has a theory on why christianity eventually overtook the roman empire. He says that unlike all of the other religions in the romans, christianity was evangelical and it was exclusive. And so every new christian wasn't just like a pagan adopting one additional god. But now, like they weren't a pagan any more, tha, you won't be. Weren't aallowed to keep your old gods as well,. right? Yea, ye. Yes.