Question: But in order that this sacrifice be powerful enough for a complete, eternal reconciliation of humanity with God and that it thereby establish a lasting, eternal covenant, was it necessary for the One who would give Himself as a sacrifice for us to be truly human, like us in all things except sin, and at the same time true eternal God?
Answer: Yes, it was necessary. For in the first place, God did not want to enter into a kind of reconciliation in which He would be found to be untruthful or unjust. Thus the truthfulness of God, who said, “In the day that you eat of the forbidden fruit you shall die” (Genesis 2:17), demanded that the mediator be the seed of the woman, that is, truly human in body and soul, who could satisfy this unchangeable truth of God and take the pain of eternal death upon Himself. (Hebrews 2:9 ff., 2 Corinthians 5:21)
9 But we do see Him who was made for a little while lower than the angels, namely, Jesus, because of the suffering of death crowned with glory and honor, so that by the grace of God He might taste death for everyone.
10 For it was fitting for Him, for whom are all things, and through whom are all things, in bringing many sons to glory, to perfect the author of their salvation through sufferings. 11 For both He who sanctifies and those who are sanctified are all from one Father; for which reason He is not ashamed to call them brethren, 12 saying,
“I will proclaim Your name to My brethren,
In the midst of the congregation I will sing Your praise.”
13 And again,
“I will put My trust in Him.”
“Behold, I and the children whom God has given Me.” Hebrews 2:9-13
20 Therefore, we are ambassadors for Christ, as though God were making an appeal through us; we beg you on behalf of Christ, be reconciled to God. 21 He made Him who knew no sin to be sin on our behalf, so that we might become the righteousness of God in Him. 2 Corinthians 5:20, 21
Moreover, the justice of God, who said, “I will not justify the wicked” (Exodus 23:7), demanded that He not establish and friendship with the wicked and unrighteous unless the just (namely, Christ) die for the unjust (1 Peter 3:18). “18 For Christ also died for sins once for all, the just for the unjust, so that He might bring us to God, having been put to death in the flesh, but made alive in the spirit.” Accordingly, the mediator who would offer Himself as a sacrifice for all had to be not only truly human but also truly God. Why?
Since the land of Egypt;
And you were not to know any god except Me,
For there is no savior besides Me. Hosea 13:4
12 Now if Christ is preached, that He has been raised from the dead, how do some among you say that there is no resurrection of the dead? 13 But if there is no resurrection of the dead, not even Christ has been raised; 14 and if Christ has not been raised, then our preaching is vain, your faith also is vain. 15 Moreover we are even found to be false witnesses of God, because we testified against God that He raised Christ, whom He did not raise, if in fact the dead are not raised. 16 For if the dead are not raised, not even Christ has been raised; 17 and if Christ has not been raised, your faith is worthless; you are still in your sins. 18 Then those also who have fallen asleep in Christ have perished. 19 If we have hoped in Christ in this life only, we are of all men most to be pitied. 1 Corinthians 15:12-19
Second, He had to take upon Himself unbearable pain for all creatures, since He had to satisfy the just judgment of God for the innumerable sins we have committed against God’s infinite majesty. Therefore, He had to be truly human in such a way that at the same time He was infinite, that is, true God. This was so that His human nature, maintained by the infinite power of His divinity, might bear and overcome the weight of God’s eternal wrath and thereby earn for us God’s grace and an infinite, everlasting payment for our sin (Hebrews 7:15 ff.; John 10:14 ff.)
15 And this is clearer still, if another priest arises according to the likeness of Melchizedek, 16 who has become such not on the basis of a law of physical requirement, but according to the power of an indestructible life. 17 For it is attested of Him,
“You are a priest forever
According to the order of Melchizedek.” Hebrews 7:15-17
14 I am the good shepherd, and I know My own and My own know Me, 15 even as the Father knows Me and I know the Father; and I lay down My life for the sheep. 16 I have other sheep, which are not of this fold; I must bring them also, and they will hear My voice; and they will become one flock with one shepherd. 17 For this reason the Father loves Me, because I lay down My life so that I may take it again. 18 No one has taken it away from Me, but I lay it down on My own initiative. I have authority to lay it down, and I have authority to take it up again. This commandment I received from My Father.” John 10:14-18
But this will be taught in greater depth when we treat the article of the creed, “Conceived by the Holy Spirit, born of the Virgin Mary.”
Caspar Olevianus, A Firm Foundation, pp. 6, 7