The New Testament reveals that in Christ's resurrection, He was born as the firstborn Son of God. Acts 13:33 says, "God has fully fulfilled this promise to us their children in raising up Jesus, as it is also written in the second psalm, You are My Son; today I have begotten You." Furthermore, Romans 8:29 refers to Christ as God's Son, the Firstborn among many brothers. From these two verses we can see that on the day of resurrection, Jesus was begotten by God to be the firstborn Son of God.
Of course, Christ's incarnation was also a birth, but that birth made Him the Son of Man. Christ did not become the Son of God through incarnation. In eternity past, before His incarnation and before His resurrection, Christ was already the Son of God. The Bible reveals that Christ, the Son of God, is eternal. God is triune—the Father, the Son, and the Spirit—and all Three are eternal. God the Father is eternal (Isa. 9:6), God the Son is eternal (Heb. 7:3), and God the Spirit is eternal (Heb. 9:14).
To be eternal means to have no beginning and no ending. Students of the Bible sometimes use a circle to signify eternity. A circle has no beginning and no ending, and it is difficult to tell whether one point on a circle comes before or after another point. In the same way, the Father, the Son, and the Spirit are all eternal, having no beginning nor ending. Hebrews 7:3 tells us that the Son of God is eternal, having neither beginning of days nor end of life. The revelation in the Bible is not that the Father existed before the Son, that the Son came into being after the Father, or that the Spirit came after the Son. Rather, the Bible says that all Three are eternal.
The Son of God is eternal, yet this eternal Son of God was born as the Son of Man about two thousand years ago. In His incarnation He was born of Mary, and by that birth He became the Son of Man. Therefore, His incarnation is His first birth. But the Bible also tells us that Christ had a second birth. In His first birth Christ was born as the Son of Man, and in His second birth He was born as the firstborn Son of God. On the one hand, John 3:16 says, "For God so loved the world that He gave His only begotten Son." This verse indicates that Christ was God's only Son. On the other hand, Romans 8:29 says, "...that He should be the Firstborn among many brothers." Have you ever considered that Christ is the Son of God in two ways? In the first way He was God's only begotten Son, and in the second way He is the firstborn Son among many sons. Romans 8:29 says that the believers are to be conformed, not to the image of the only begotten Son, but to the image of God's firstborn Son.
At this point we need to ask ourselves what the difference is between the only begotten Son and the firstborn Son. Our first response may be to say that the only begotten Son had no brothers, but the firstborn Son has many brothers. Although this is true, we still need to ask what the difference is in the Son of God Himself. The difference between the only begotten Son of God in eternity past and the firstborn Son of God in resurrection is that in eternity past, before His incarnation, He possessed only divinity without humanity. But through the process of incarnation, He put on humanity. He passed through human living, entered into death, and came out in resurrection. In resurrection He still remained the Son of God according to His divinity, but there was something more; He also possessed the humanity He obtained through incarnation. The humanity He put on in incarnation was also brought into resurrection to share in the sonship. This is why Acts 13:33 says that on the day of resurrection, Christ was begotten of God to be God's Son. It means that resurrection "sonized" His humanity, made it also the Son of God. According to Acts 13:33, Christ's resurrection was a birth, making Him not only God's only begotten Son with divinity, but also God's firstborn Son with both divinity and humanity.