Rome was a vast empire that covered Europe. There was one united political Roman State but there was a multitude of religions in that vast area. These religions fought with each other and they were violent fights involving deaths. Many people were involved and the Romans had trouble controlling the crowds.
Constantine had the solution; to have an officially state sanctioned religion. He would control what the religion would be. The Church precepts would support the Roman State. It would pacify the people. Any challenging religions that would compete would be wiped out. The people he put in charge of this religion was a pagan group that had pagan ceremonies on capitol hill outside of city of Rome in what today is known as St. Peters Square. Today those same grounds are known as the Vatican City, and the group is known as the Catholics.