Sonship and the Resurrection

Concerning His Son Jesus Christ our Lord, who was born of the seed of David according to the flesh, and declared to be the Son of God with power according to the Spirit of holiness, by the resurrection from the dead. (Rom 1:3-4 NKJ)

The declaration of the Christ as the Son of God with power according to the Spirit of holiness, by the resurrection from the dead was foreshadowed in the baptism of Jesus, which was in itself a foreshadowing of His death, burial, and resurrection; for that in essence is what baptism signifies. It is not a coincidence then that we hear the first heavenly announcement that Jesus is the beloved Son of God after Jesus comes out of the waters of baptism.
When He had been baptized, Jesus came up immediately from the water; and behold, the heavens were opened to Him, and He saw the Spirit of God descending like a dove and alighting upon Him. And suddenly a voice came from heaven, saying, "This is My beloved Son, in whom I am well pleased." (Mat 3:16-17 NKJ)
Therefore, those who are baptized into Christ are promised:
Or do you not know that as many of us as were baptized into Christ Jesus were baptized into His death? Therefore we were buried with Him through baptism into death, that just as Christ was raised from the dead by the glory of the Father, even so we also should walk in newness of life. For if we have been united together in the likeness of His death, certainly we also shall be in the likeness of His resurrection. (Rom 6:3-5 NKJ)
Jesus didn't become the Son at His resurrection; for, He is Son eternally. This is confirmed by the heavenly voice at His baptism. For the Son came to offer His body once for the sins of the world, but He rose, and He lives forever; and His eternal priesthood is after the power of an endless life (Heb.7: 16-25). 

There is a second time when the voice is heard again, and this is at the transfiguration of Jesus on the Mount.
Now after six days Jesus took Peter, James, and John his brother, led them up on a high mountain by themselves; and He was transfigured before them. His face shone like the sun, and His clothes became as white as the light. And behold, Moses and Elijah appeared to them, talking with Him. Then Peter answered and said to Jesus, "Lord, it is good for us to be here; if You wish, let us make here three tabernacles: one for You, one for Moses, and one for Elijah." While he was still speaking, behold, a bright cloud overshadowed them; and suddenly a voice came out of the cloud, saying, "This is My beloved Son, in whom I am well pleased. Hear Him!" And when the disciples heard it, they fell on their faces and were greatly afraid. (Mat 17:1-6 NKJ)
Peter had to understand that tabernacles of earth was not the end goal of God for man.
For we know that if our earthly house, this tent, is destroyed, we have a building from God, a house not made with hands, eternal in the heavens. For in this we groan, earnestly desiring to be clothed with our habitation which is from heaven, if indeed, having been clothed, we shall not be found naked. For we who are in this tent groan, being burdened, not because we want to be unclothed, but further clothed, that mortality may be swallowed up by life. Now He who has prepared us for this very thing is God, who also has given us the Spirit as a guarantee. (2Co 5:1-5 NKJ)
For our citizenship is in heaven, from which we also eagerly wait for the Savior, the Lord Jesus Christ, who will transform our lowly body that it may be conformed to His glorious body, according to the working by which He is able even to subdue all things to Himself. (Phi 3:20-21 NKJ)
Undoubtedly, the transfiguration of Christ was only a glimpse into the glory of the Son, and a foreshadowing of the transfiguration of believers at the coming of Christ, for "we shall be like Him, for we shall see Him as He is." (1Jo 3:2)
Behold, I tell you a mystery: We shall not all sleep, but we shall all be changed--in a moment, in the twinkling of an eye, at the last trumpet. For the trumpet will sound, and the dead will be raised incorruptible, and we shall be changed. For this corruptible must put on incorruption, and this mortal must put on immortality. So when this corruptible has put on incorruption, and this mortal has put on immortality, then shall be brought to pass the saying that is written: "Death is swallowed up in victory." "O Death, where is your sting? O Hades, where is your victory?" (1Co 15:51-55 NKJ)
This final redemption of the body, the glorification of the believers, is the point when creation will see the glorious manifestation of the sons of God, for "whom He foreknew, He also predestined to be conformed to the image of His Son, that He might be the firstborn among many brethren." (Rom 8:29 NKJ)
The Spirit Himself bears witness with our spirit that we are children of God, and if children, then heirs-- heirs of God and joint heirs with Christ, if indeed we suffer with Him, that we may also be glorified together. For I consider that the sufferings of this present time are not worthy to be compared with the glory which shall be revealed in us. For the earnest expectation of the creation eagerly waits for the revealing of the sons of God. For the creation was subjected to futility, not willingly, but because of Him who subjected it in hope; because the creation itself also will be delivered from the bondage of corruption into the glorious liberty of the children of God. For we know that the whole creation groans and labors with birth pangs together until now. Not only that, but we also who have the firstfruits of the Spirit, even we ourselves groan within ourselves, eagerly waiting for the adoption, the redemption of our body. (Rom 8:16-23 NKJ)