16. Why must He be a true and righteous man?

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: 5:15.  [2]Isa. 53:3-5.  

17. Why must He also be true God?

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 2:24.  [2]Jn. 3:16;  Acts 20:28.  [3]I Jn. 1:2 .  

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

Our Lord Jesus Christ, [1] who is freely given unto us for complete redemption and righteousness. [2]

[1] Mt. 1:23;   I Tim. 3:16;   Lk. 2:11.   [2]I Cor. 1:30;   *Acts 4:12.  

19. From where do you know this? 

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. 3:15.  [2]Gen. 22:18; 49:10-11;   Rom. 1:2;   Heb. 1:1;   Acts 3:22-24; 10:43.  [3]Jn. 5:46.  Heb. 10:7.  [4]Rom. 10:4;  Gal. 4:4-5; *Heb. 10:1.