The Catholic Church became the official religion of the Roman Empire when the Roman Emperor Constantine converted to it. This opened the door for many changes in the Empire, for example, a church hierarchy was established and Rome became the center of the Catholic Church.
Because of the length of the Roman Empire, the Catholic Church was able to have a big influence on the continent. Churches often became the centers of the cities, they influenced greatly on costumes and cultures, architecture, food and others.
Later, this opened the door for the inquisition in Europe what changed and persecuted many people who were not catholic.