Constantine was a Roman Emperor who was fighting rivals for power over Rome. Before battle, he was praying, and saw a cross, a symbol of Christianity. He ordered that the sign be painted onto the soldier's helmets. Constantine won the battle. He then ordered for the persecution of the Christians to end.
Jesus told Peter that he would be the first Bishop of Rome. Peter would travel and spread the word.
Soon every city had a Bishop, and they all claimed they were the successor to Peter. They called Peter the first Pope, leader of the Church.