Part of his influence in decreeing the Catholic religion as the state religion came from his mother, Helena, who had converted to Christ sometime before. She was implicated in the murder of Constantine's step mother and was forced to flee and live in exile. She went on a pilgrimage to the holy land. There, by a miracle, she found the tomb of Jesus. She also, by a miracle found the birth place of Jesus. She also, by a miracle, found the remains of the three crosses that were used to crucify Jesus and the other two thieves. This discovery was truly miraculous since Rome had crucified thousands of Jews in the 300 years since the death of Jesus to the time of her discovery.
When Constantine heard of his mother's discoveries, he had churches built on the sites and to this day people go to these places for reverence. His mother sent pieces of the crosses all across the Empire and are still seen in churches today. The Catholic Church was so impressed by her that they made her a Saint and labeled her "Saint Helena, Discoverer of the True Cross."