He didn't interfere with the Church's teaching, but he really changed the day-to-day lives of pastors and most lay believers by legalizing Christianity.
It's hard for us to imagine today a shift from the psychological pressure of being hunted for one's beliefs to the relief of being welcomed and encouraged.
His influence was not direct, but it was enormous: there were now no new martyrs. There were plenty of places to meet and teach when before there were few. Christians who had to sacrifice their inheritances and livelihoods and social position no longer had to.
Probably there were many new converts who were not particularly sincere or committed, but who were motivated to get on the Emperor's good side.
He definitely changed the dynamic drastically.
Was he (Constantine) responsible for the building of St. Peter’s Basilica? And the first ecumenical council at Nicea?