Heidelberg Catechism - Lord's Day 6

16. Q . Why must the mediator be a true and righteous human?
     A. God’s justice demands that human nature, which has sinned, must pay for sin; but a sinful human could never pay for others.

17. Q . Why must the mediator also be true God?
     A. So that the mediator, by the power of his divinity, might bear the weight of God’s wrath in his humanity and earn for us and restore to us righteousness and life.

18. Q . Then who is this mediator— true God and at the same time a true and righteous human?
     A. Our Lord Jesus Christ, who was given to us to completely deliver us and make us right with God.

19. Q . How do you come to know this?
     A. The holy gospel tells me. God began to reveal the gospel already in Paradise; later God proclaimed it by the holy patriarchs and prophets and foreshadowed it by the sacrifices and other ceremonies of the law; and finally God fulfilled it through his own beloved Son.