Constantine was into Sol Invictus (Mithras the Sun god, he of the Heavenly Bull) before he became a Christian.
Other coincidences.
- Sol Invictus was a trifold unity, consisting of the rising sun, the midday sun, and the setting sun, who were three, yet one.
- Mithras birthday was December 25th.
- He was a virgin birth, born in a cave, placed in a manger, and tended to by shepherds. (Early pictures of the Virgin Mary often depict her holding baby Jesus in a cave.)
- Mithra worshippers would consume the blood and flesh of Mithra in the form of bread and wine. (only the men though. No women allowed.)
- "In Hoc Signo Victis" signifies a Sun Cross.
Politics:
After the great Jewish revolts, Christianity and Mithraism became the two leading religions of the Roman Empire. Emperors could rise or fall depending on which religion they supported. Amalgamizing the two would have made Constantine's rule stable, and ended the religious unrest of the first centuries.
Of course these are just observations. Faith is personal.