Emperor Constantine is known for his conversion to Christianity. Before his battle with Lactantius at the Milvian bridge (October 28, 312), he saw a sign of the chi rho in the sky, which are the first two letters of the name Christ. He also heard a voice saying that he would win under this sign. As it turned out, Constantine did win the battle and he attributed his victory to the Christian God. For this reason, he converted to Christianity and with him most of the Empire.
He also moved the capital of the Empire to a new place, Constantinople. This was a huge endeavor. There were huge churches that were built there. This religious awakening, in turn, created a new type of art and architecture. In short, it was Constantine's converstion to Christianity that started everything.