Christianity began as a religion based on the teachings of Jesus Christ, who taught that there is one God. In the early centuries AD, Christians faced persecution from Roman emperors like Nero. The Colosseum was built in 80 AD and used for gladiator fights and public entertainment. Despite threats of death, many early Christians refused to deny their faith, including Polycarp who was martyred. The Roman emperor Constantine ended persecutions in the 4th century AD and allowed Christianity to spread. During the Middle Ages from 500-1500 AD, the Catholic Church became very powerful and influential in Europe under the leadership of the Pope in Rome.