Lord's Day 6: The Only Mediator

Q16. Why must the mediator be a true and righteous human?

God's justice demands that human nature, which has sinned, must pay for sin; but a sinful human could never pay for others.

Hebrews 2:14-17

"Since therefore the children share in flesh and blood, he himself likewise partook of the same things, that through death he might destroy the one who has the power of death, that is, the devil, and deliver all those who through fear of death were subject to lifelong slavery. For surely it is not angels that he helps, but he helps the offspring of Abraham. Therefore he had to be made like his brothers in every respect, so that he might become a merciful and faithful high priest in the service of God, to make propitiation for the sins of the people."

Zacharias Ursinus on Q16: "The Mediator must be truly human because divine justice required that the same nature which sinned should make satisfaction. A human sinned; therefore a human must pay. But this human must also be righteous—for a sinner cannot satisfy for sin, since he already owes obedience for himself."

— Commentary on the Heidelberg Catechism (1591)