1. Official persecution was lifted and being a Christian — at least in name — became popular and prudent.
2. Constantine left Rome and built a “new Rome” in the East as the new imperial capital of the empire. He chose the city of Byzantium (today’s Istanbul in Turkey) and renamed it after himself: Constantinople.
3. The most divisive schism the Christian church had ever experienced broke out within Constantine’s reign.
4. The church held its first ecumenical (universal) council to settle doctrinal and ecclesiastical conflicts: the Council of Nicea.

To read more on this topic (and many more) let me encourage you to purchase Roger Olson’s book The Story of Christian Theology. You can do so here.