They became Romans

It was a small persecuted community in Palestine upholding strict monotheism, rejecting idolatry, and awaiting and working for divine justice. And when Rome co opted their faith, Rome didn't convert to their teachings, it converted their teachings into something else entirely. The Romans didn't become Christians, the Christians became Roman. The radical anti imperial message of Isa Alaihi Salam, a prophet who condemned excessive wealth, power, hypocrisy, that message was gutted and remade into a state religion that sanctified wealth, it sanctified power and hypocrisy. Those pagan idolaters changed monotheism into trinity.

The uncompromising tawhid, the oneness of Allah that was preached by Isa was replaced with a convoluted theology borrowed from Greek philosophy, borrowed from pagan mystery, mystical cults, you know. The simple prophetic message of submission to Allah was buried beneath layers upon layers upon layers of metaphysical abstraction until the man who had preached according to their own book, the Lord your God is one God was himself declared to be a God. It went from an anti empire message to empire's handbook. Oh, the religion of the oppressed became the religion of the oppressor.