At the Battle of Milvian Bridge October 28, 312, Constantine was successful mainly due to his superior military skills and the incompetence of the engineers used by Maxentius for construction of the temporary wooden bridge.
Constantine did not establish Christianity, although the Edict of Milan gave religious freedom. The god Sol Invictus, the Unconquered Sun, featured prominently on Constantinian coins and monuments in the years before and after the battle indicating that he kept several beliefs as a hedging strategy.
Constantine was responsible for switching the day of rest from Saturday (Sabbath) to Sunday, the first day of the week in the Jewish Calendar, as well as the day of the Resurrection, and the day which the Roman state had dedicated to Sol Invictus (hence Sun-day), the main rival religion to Christianity at the time, and of which Constantine was chief priest (pontifex maximus) until his death.
http://en.wikipedia.org/wiki/Constantine_I_and_Christianity
Many still erroneously believe today that the Edict established Christianity as the sole faith of Rome, a change which didn't take place until the early 390s, under Theodosius I.