Why do christology? H. R. Mackintosh has 4 reasons (via Purves)
Why do Christology? Mackintosh suggests that four motives may be found in the New Testament itself. (1) It was believed that Jesus was the fulfillment of Old Testament prophecy, and that God’s revelation ended with him. If so, who was / is he? (2) Jesus’ exaltation and his gift of the Spirit mean that he is Lord, begetting in believers a transcendent life and a hope in his coming again to be revealed as central and omnipotent. If so, who was / is he? (3) The apostolic church, extending the mission beyond the Jewish circle, discovered that Jesus was for the whole world. His significance was universal. If so, who was / is he? (4) The self-witness of Jesus quickened the thought of his awareness of a unique sonship, which raised all manner of questions concerning his relationship to God. If so, who was / is he?