Henry VIII changed the religion of England because the Catholic church would not grant him a divorce from Catherine of Aragon. When Pope Clement VII refused to grant a special dispensation to annul the marriage, Henry got the Archbishop of Canterbury to comply and declared himself head of the church.
Henry VIII married Catherine of Aragon in 1509, but by the 1520s, the king still lacked a male heir. Henry decided he wanted the marriage annulled, using the argument that since Catherine had been his brother's widow, it had been a union in violation of the laws of the Bible and should be annulled. However, Pope Julius II had given Henry a papal dispensation to be married in the first place, so Pope Clement VII refused to contradict that dispensation and declare the marriage invalid. An enraged Henry secured his divorce from a friendly Archbishop and then moved against the entire religious establishment.
Henry used the Statute of Praemunire, an act passed in 1392 that forbade allegiance to foreign rulers or the Pope, to demand loyalty from the clergy in England. He demanded a massive payment in exchange for a "pardon" of their crimes, and in the process had himself declared the head of the church. Over the remainder of his reign, Henry steadily destroyed the Roman Catholic church's power in England and established a new faith.