16. Why must [our mediator] be truly human and truly righteous?
God’s justice demands that human nature, which has sinned, must pay for its sin; but a sinner could never pay for others.
Ro 5.12,15; 1Co 15.21; Heb 2.14-16
Heb 7.26-27; 1Pe 3.18
17. Why must he also be true God?
So that, by the power of his divinity, he might bear the weight of God’s anger in his humanity and earn for us and restore to us righteousness and life.
Isa 53; Jn 3.16; 2Co 5.21
18. And who is this mediator–true God and at the same time truly human and truly righteous?
Our Lord Jesus Christ, who was given us to set us completely free and to make us right with God.
Mt 1.21-23; Lk 2.11; 1Ti 2.5
19. How do you come to know this?
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; finally, he fulfilled it through his own dear Son.
Ge 22.18; 49.10
Isa 53; Jer 23.5-6; Mic 7.18-20; Ac 10.43; Heb 1.1-2
Lev 1-7; Jn 5.46; Heb 10.1-10
Ro 10.4; Gal 4.4-5; Col 2.17
From the Heidelberg Catechism.