In 2 Samuel 7:12 God was actually telling David that instead of building something for God, he needed God to build His Son into him. It is a marvelous fact that eventually David's seed would be called the Son of God (Rom. 1:3). This indicates the principle of incarnation. Christ is the God-man. As God, He is the Son of God; as man, He is our seed. If the Son of God is not constituted into us, we are nothing. The Son of God must become our seed, yet He remains the Son of God.
When we were saved, many of us believed the gospel in a shallow way. The more I study the Bible, the more I realize that the gospel is not shallow. The genuine and intrinsic gospel is that God intends to work Himself in Christ into us. In 2 Samuel 7:12 we have the genuine preaching of the gospel. If we preach this intrinsic gospel, we will tell others that they should not think that they can do something for God, for their human life is empty. We will tell them that they are short of something and that what they need is Christ, the very God Himself, to be wrought into their being. This is the gospel.
In order to work Himself into us, God first became a man through incarnation. As a man He died for us, and then rose up from among the dead and became the life-giving Spirit (1 Cor. 15:45b). This life-giving Spirit is our Savior, who redeemed us on the cross and who is now saving us in our spirit. He is our Redeemer, Savior, life, and seed. As such a One, He is now being built, constituted, into us, making us a part of Him, even as He has made Himself a part of us.
If we read 2 Samuel 7 under God's enlightening, we will see that the seed of David is Christ. According to verse 7 God will establish David's kingdom, which refers to Christ's kingdom. This means that the kingdom of David is Christ's kingdom. The prophets spoke of David and Christ as one. In Hosea 3:5 and Amos 9:11 the coming of King David refers to Christ's coming.
Second Samuel 7:13 reveals that it is Christ who will build a house for God's name, and God will establish the throne of Christ's kingdom forever.
Verse 14a says, "I will be his Father, and he will be My son." This actually reveals that God will be Christ's Father and that Christ will be God's Son (Heb. 1:5b). God's salvation is just to make Christ, His embodiment, ours. The Son of God, who is the embodiment of God, God Himself, is in us. Therefore, we can declare that Christ, the Son of God, has been built into us.