Q. 16. Why must he be very man, and also perfectly righteous?
Because the justice of God requires that the same human nature which hath sinned, should likewise make satisfaction for sin; and one, who is himself a sinner, cannot satisfy for others.
 Romans 5:12, 15;  1 Peter 3:18; Isaiah 53:11.
Q. 17. Why must He in one person be also very God?
That He might, by the power of His Godhead, sustain in His human nature the burden of God’s wrath; and might obtain for, and restore to us, righteousness and life.
 1 Peter 3:18; Acts 2:24; Isaiah 53:8;  1 John 1:2; Jeremiah 23:6; 2 Timothy 1:10; John 6:51.
Q. 18.Who then is that Mediator, who is in one person both very God, and a real righteous man?
Our Lord Jesus Christ: “who of God is made unto us wisdom, and righteousness, and sanctification, and redemption.”
 Matthew 1:23; 1 Timothy 3:16; Luke 2:11;  1 Corinthians 1:30.
Q. 19. Whence knowest thou this?
From the holy gospel, which God Himself first revealed in Paradise; and afterwards published by the patriarchs and prophets, and represented by the sacrifices and other ceremonies of the law; and lastly, has fulfilled it by His only begotten Son.
 Genesis 3:15;  Genesis 22:17–18 and 28:14; Romans 1:2; Hebrews 1:1; John 5:46;  Hebrews 10:7–8;  Romans 10:4; Hebrews 13:8.
Last Lord’s Day, we saw how fallen man is unable to make satisfaction for his own sin, and concluded by anticipating the answers to this week’s questions that the only mediator between God and man, who is able to make satisfaction for our sin, is the Lord Jesus Christ, who is the eternal Son of God, who took on human nature in order to bear our sin on our behalf. Christ has to be fully man because only one who has the same nature as man can truly represent man. Additionally, He must be perfectly guiltless and righteous because otherwise He will have to pay for His own sin, and so can never satisfy for others. It is for this reason that the Lord Jesus Christ was born of a virgin, with the “power of the Highest” overshadowing His incarnational mother (Lk 1:35), so that Adam’s guilt is not imputed on Him, nor Adam’s corruption inherited by Him.
Heidelberg Catechism >