Christianity began as a Jewish religious sect that followed the teachings of Jesus Christ in the 1st century CE. The movement grew with the missionary efforts of Paul and others following Jesus' resurrection. Disagreements over the nature of Jesus led to divisions in the early church. By the 4th century, Constantine's conversion marked a shift that led to Christianity becoming the dominant religion in the Roman Empire. However, theological differences continued and grew over time, culminating in the Great Schism of 1054 and the division between the Catholic and Orthodox churches. The Protestant Reformation in the 16th century introduced further divisions with reformers such as Martin Luther and John Calvin challenging Catholic doctrine.