God could become man for the same reason Jesus could perform miracles and God could create the universe out of nothing: namely, that God can do as He pleases. Anything that is not immoral that can possibly be done, can be done by God. “With God all things are possible”” (Matthew 19:26).
He announced His coming (Old Testament prophecies). Then when He did come, He told us exactly who He was. For example, “Before Abraham was born, I am” (John 8:58), “The Father and I are one” (John 10:31), and “Anyone who has seen Me has seen the Father” (John 14:9). To make His point clear, Jesus performed miracles that no other human being has performed, the culmination of which was His resurrection from the dead.
We can think of Jesus as the invisible God taking on a visible form. He did this in order to communicate His love to us, to show us the way to Himself, and ultimately to provide the way to Himself. Jesus did this by allowing Himself to be put to death, crucified, for the forgiveness of our sins. The Bible says that, in Jesus, God “made himself nothing, taking the very nature of a servant, being made in human likeness. And being found in appearance as a man, he humbled himself and became obedient to death–even death on a cross” (Philippians 2:5).
In Jesus, the transcendent God ‘untranscended’ so that He could die in our place, as a bridge for us to Himself, God. Here are some other ways that the Bible explains this remarkable event and Person…
In the beginning was the Word, and the Word was with God, and the Word was God…The Word became flesh and made his dwelling among us. (John 1:1,14)
He [Jesus Christ] is the image of the invisible God. (Colossians 1:15)
In Christ all the fullness of the Deity lives in bodily form. (Colossians 2:9)
The Son is the radiance of God’s glory and the exact representation of his being. (Hebrews 1:3)
The official teaching, called doctrine, is that Jesus is the second person of the Trininy, became flesh for us and for our sake. He will always be in this spiritual/phisical form. And someday, we will be like him.