7. 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 for2 and restore to us righteousness and life.3
1 Isaiah 53:8. Acts 2:24. 2 John 3:16. Acts 20:28. 3 I John 1:2.
The premise of the question is that God the Son is “the only-begotten Son of God, the begotten of the Father before all worlds; God of God, Light of Light, very God of very God; begotten not made, being of the same substance with the Father, by whom all things were made.” This great mystery, the incarnation of God the Son, is at the heart of our faith and it is so because it at the heart of the biblical account of who Jesus is.
The Christian confession that Jesus is “true God” means that there is an antithesis between the biblical, orthodox faith and other views of Jesus’ deity. For one thing, we deny all those views that teach or imply that Jesus is merely like God but not of the same substance or essence as the Father and the Spirit. We deny that God the Son became God.
One aspect of the mystery of the Son’s deity is that he is eternally begotten. There never was when the Son was not and yet he has always been the Son. There never will be when the Son is not. He is co-essential with the Father the Spirit. He is co-eternal with the Father and the Spirit and yet he is personally distinct from the Father and the Spirit. As the Athanasian Creed says,
we worship one God in Trinity, and Trinity in Unity; Neither confounding the persons, nor dividing the Substance. For there is one Person of the Father, another of the Son, and another of the Holy Spirit. But the Godhead of the Father, of the Son, and of the Holy Spirit, is all one, the Glory equal, the Majesty co-eternal. Such as the Father is, such is the Son, and such is the Holy Spirit.
The catholic church, including the Reformed Churches, understands God’s Word to teach that Jesus did not become God. He was not adopted. The Father did not look down, as it were, and see that Jesus was or would become a good man and adopt him into the deity. In the incarnation the eternal Son of God, who was in the beginning, who was with God, who was God, who is God, that very same consubstantial, co-eternal person, became incarnate of the virgin Mary.
To say that he was, is, and shall be true God is to say that he did not merely appear to be God (or man). The true second person of the Holy Trinity took on true humanity. That person, through whom all things came into being and without whom nothing came into being that has come into being, that one entered history in the incarnation.
When we meditate, even if only briefly, on the true divinity of God the Son, then we appreciate just a little more what it means to say, “who for us men and for our salvation….” We understand why the early church confessed that the virgin Mary was “theotokos,” i.e., the God-bearer. The child in her womb, who had an umbilical cord, who was truly human, like us in every respect, sin excepted, was also true, eternally begotten God. The deity of Jesus is not a theory, a postulate, a speculation, but a revealed truth at the very heart of the biblical revelation (more on that later) and thus at the center of the historic confession of all Christian churches in all times and places.