The Heidelberg Catechism, This Lord's Day week 6

Q16: Why must He be a true and righteous man?

A16: Because the justice of God requires [1] that the same human nature which has sinned should make satisfaction for sin; but one who is himself a sinner cannot satisfy for others.[2]
1. Rom. 5:152. Isa. 53:3-5
Q17: Why must He also be true God?

A17: That by the power of His Godhead He might bear in His manhood the burden of God's wrath,[1] and so obtain for [2] and restore to us righteousness and life.[3]
1. Isa. 53:8; Acts 2:242. John 3:16; Acts 20:283. I John 1:2

Q18: But who now is that Mediator, who in one person is true God and also a true and righteous man?

A18: Our Lord Jesus Christ,[1] who is freely given unto us for complete redemption and righteousness.[2]
1. Matt. 1:23; I Tim. 3:16; Luke 2:112. I Cor. 1:30; Acts 4:12

Q19: From where do you know this?

A19: From the Holy Gospel, which God Himself first revealed in Paradise,[1] afterwards proclaimed by the holy Patriarchs [2] and Prophets, and foreshadowed by the sacrifices and other ceremonies of the law,[3] and finally fulfilled by His well-beloved Son.[4]
1. Gen. 3:152. Gen. 22:18; 49:10-11; Rom. 1:2; Heb. 1:1; Acts 3:22-24; 10:433. John 5:46; Heb. 10:74. Rom. 10:4; Gal. 4:4-5; Heb. 10:1