Christianity is the largest religion in Germany. It was introduced to the area of modern Germany by 300 AD, while parts of that area belonged to the Roman Empire, and later, when Franks and other Germanic tribes converted to Christianity from the fifth century onwards. The area became fully Christianized by the time of Charlemagne in the eighth and ninth century. After the Reformation started by Martin Luther in the early 16th century, many people left the Catholic Church and became Protestant, mainly Lutheran and Reformed. In the 17th and 18th centuries, German cities also became hubs of heretical and sometimes anti-religious freethinking, challenging the influence of religion and contributing to the spread of secular thinking about morality across Germany and Europe.