After killing martyr after martyr after martyr, the Romans saw in this belief something useful that they could control.
Before converting to it, they standardized it and shaped it to their convenience in the First Council of Nicaea. They also decided which books made it into the bible, and which ones were "apocryphal".
The Bishop of Rome became the Pope, and after the fall of the Roman empire, the Vatican still exerted a tremendous control: the pope could excommunicate anyone, including kings.
We still have records of what happened at Nicaea. Basically they standardised the date of Easter and condemned Arianism (against the will of Constantine who wanted a compromise position).
The Council of Nicaea actually didn't have anything to do with the Bible, despite what a lot of people say. By that point the Church had more or less reached a consensus on the canon after a couple hundred years of debate.
"They also decided which books made it into the bible, and which ones were "apocryphal"
That's a bit far fetched claim. Apocryphal books are apocryphal for a reason, they didn't make into the canon as they couldn't have been written by witnesses of what happened at the beginning of our era - mentioned geographical places are wrong, mentioned historical events and people are messed up, language is wrong, etc.
You're not actually discrediting the claim that they 'decided which books made it into the bible, and which ones were "apocryphal"', only offering a rationale for doing so.
It remains true that they decided which parts of existing and accepted Christian doctrine were true and which parts were no longer true.
