In AD 321 when he decreed that Sunday would be a day of rest, his intention was to honor the god Mithras, the Unconquered Sun, hence the name "Sun" day. He functioned as the high priest of paganism, retaining the pagan title, Pontifex Maximus which means chief of the pagan priests. When he dedicated Constantinople as his new capital in 330 AD, he decorated it with treasures taken from heathen temples, and he used pagan magic formulas to protect crops and heal diseases.
This flies in the face of everything Jesus tried to teach us, that we are the church, that only God is holy, and that we are the living temple of God's Spirit. Constantine's negative influence on Christianity cannot be overstated. By the third century, Christianity had borrowed so much from the heathen culture that it changed the face of our faith forever. No longer was the church a group of Holy Spirit filled people meeting together to worship god. It now became, much like the pagan temples, a physical building made with hands where they believed God dwelt in a special way.
Taken from Pagan Christianity by Frank Viola and George Barna