Since as early as 230 A.D., Christians referred to Mary as Theotokos, which means: "God bearer". By the fifth century, there were some people who questioned this title, claiming that Mary was only the mother of Jesus' human nature. The bishops gathered at the Council of Ephesus (431 A.D.) and the Council of Chalcedon (451 A.D.) taught definitively that Christ is fully God and human and that these two natures are united in one person, Jesus Christ. Therefore Mary could be called "Mother of God" because she gave birth to Jesus who is fully divine as well as human.