From the outset, Christians were people with certain beliefs—especially about Jesus Christ—seen as fundamental to the existence of the church. The fourth, fifth, and sixth centuries were marked by lengthy controversies—particularly in the Eastern Church—about how Christ, the Son of God, was himself God (the doctrine of the Trinity); and how he was both man and God (the doctrine of the person of Christ, or Christology). This was a period of great importance in the formation