Constantine was not a Catholic Pope, Bishop, or anything. He was a Roman Emperor.
He DID have a conversion experience which led him to seek Christianity. At first he was merely curious about it… but largely remained pagan. Eventually some say he converted.
Either way… it ended the persecution of the Christians in the Roman Empire. He also urged the Catholic church to create a single canon of Scripture so he could study it. This resulted in the creation of the Christian Bible.
But through all of this… the Catholic church remained centered in Rome. It was not a department of the Roman Empire or subject to interference by the Emperor. It was separate.
First because the Roman government persecuted it, but secondly, because it was religious not political.
The term “Roman Catholic” wasn’t even in use at that time. That term only came into usage around the 1500s after the Protestant Reformation as a slur and an attack on the Catholic faith to try and link it to the Pagan Roman Empire.
Kinda like calling an American “Communist”