Chapter 6: Lord's Day 6
Section 16. Q. 16: Why must the mediator be truly human and truly righteous? A. God's justice demands that human nature, which has sinned, must pay for sin; but a sinner could not pay for others.
Section 17. Q. 17: Why must the mediator also be truly 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.
Section 18. Q. 18: Then who is this mediator — truly God and at the same time truly human and truly righteous? A. Our Lord Jesus Christ, who was given to us to completely deliver us and make us right with God.
Section 19. Q. 19: How do you come to know this? A. The holy gospel tells me. God himself began to reveal the gospel already in Paradise; later, he proclaimed it by the holy patriarchs and prophets, and portrayed it by the sacrifices and other ceremonies of the law; and finally, he fulfilled it through his own dear Son.