"Concerning His Son Jesus Christ our Lord, which was made 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" (Romans 1:3-4).
How Christ could be both man and God is a great mystery, but the fact that He is both man and God is certain, both from Scripture and from history. This "hypostatic |substantive| union" is set forth in many passages of Scripture, including this one. He was "made" to be a man, by placing the body of flesh "prepared" for Him by God (Hebrews 10:5) in the womb of the virgin, who was herself descended from David. But then He was "declared" to be the Son of God by His mighty resurrection from the dead, an event beyond the power of any but God Himself to accomplish.
A similar testimony is found in Galatians 4:4: "But when the fulness of the time was come, God sent forth His Son, made of a woman, made under the law." As deity, He was "sent forth" from God in heaven. In His humanity, He was "made of a woman." His "being in the form of God" preceded His taking upon Himself "the form of a servant" (Philippians 2:6-7). As the eternal Word, He "was God," but He "was made flesh, and dwelt among us" (John 1:1,14).
The same truth is found even in the Old Testament Messianic prophecies. "For unto us a child is born, unto us a Son is given" (Isaiah 9:6). As a child, He must be born, but as God's Son, He would be given. Furthermore, that human child would also be the "mighty God" and the "everlasting Father." "A virgin shall conceive, and bear a son, and shall call His name Immanuel" ("God with us" -- Isaiah 7:14).
Christ has eternally been God; He will eternally be Man. He is, forever, the God-Man -- man as God intended man to be and also the one true, eternal God. HMM