“The gospel … concerning his Son, who was born of the seed of David, according to the flesh, who was declared to be the Son of God with power, according to the spirit of holiness, by the resurrection from the dead, even Jesus Christ our Lord…” (1:1-4).
The gospel (good news) which Paul was separated to by God concerned God’s Son. Notice how Paul identifies Jesus: seed of David according to the flesh: declared… openly demonstrated… that He was the Son of God! Herein is stated the dual nature of Jesus which is universally agreed by all New Testament writers. John speaks of Jesus as being God who was made flesh (Jn. 1:1, 14). Peter confessed him to be the Son of God (Mt. 16:18). Paul alludes repeatedly in his letters to this dual character of Jesus. According to the Philippian letter, Jesus existed in the form of God and took upon himself the form of man (Phil. 2:5-8). In his first letter to Timothy Paul wrote of the “mystery of godliness… manifested in the flesh” (3:11). Jesus most often referred to himself as the “son of man” but he did not refuse nor reject the confession from others that he was the “Son of God”.
The “seed of David” according to the flesh. It was not only necessary that God should took on himself “flesh”; the “form of a servant”; to “share in flesh and blood” to accomplish his great object and intent of human redemption (Jn. 1:14; Ph. 2:5-9; Hb. 2:14); to fulfill scriptures, he must be the Son of David. God not only promised Abraham, Isaac, Jacob and Judah that through their descendants the Anointed One would come; He also gave that promise to David. In Paul’s sermon in Antioch of Pisidia he said “of this man’s seed hath God according to promise brought unto Israel a Savior, Jesus” (Acts 13:23). That promise was made to David in 2 Samuel 7:12-14. David took much comfort in that promise, although he likely did not fully understand the import of it. Many of the Psalms express the assurance of God’s promise to him, among which is Psalms 89:3. The prophets repeated this promise. Micah identified Bethlehem as the place where one would come forth from who would be a ruler of his people. Naming Bethlehem (David’s birthplace) identifies that ruler as from David’s house (Micah 5:2). Other prophets who repeated this promise are Isaiah, Jeremiah and Zechariah. The ancestry of Jesus was linked to David in both genealogies of Jesus and when Gabriel announced to Mary she would bear a son, she was told that “he shall be great and shall be called the Son of the Most High: and the Lord God shall give unto him the throne of his father David…” (Mt. 1; Luke 3:23-38; Lk.1:32). His nation knew him to be David’s seed and one of the often-made appeals to Him for help and compassion would be couched in these words “Jesus, thou son of David” (Mt. 9:27; 15:22; 20:31). That Jesus was David’s descendant simply is unassailable. It is a truth that cannot be gainsaid. However, Davidic ancestry presents nothing uncommon within itself. David had many wives and many descendants. It not only was a real possibility that David should have living descendants when Jesus lived; it is also very probable that David has living descendants today, although proving such descent would be nearly impossible.
The unusual and extremely remarkable nature of Jesus is found in Paul’s further identity of him “Who was declared to be the son of God…” (Rm. 1:4). While there are none (at least not to my knowledge) who would take issue with Jesus’ claim to Davidic ancestry, His claim to be the Son of God is hotly contested and flatly denied: denied by the atheists who say there is no God, therefore Jesus could not be God’s Son; and denied by many others who, although professing faith in Deity, are unwilling to concede such nature to Jesus. So, while Paul offered no proof for Jesus’ fleshly descent from David other than his assertion of it; he validates his claim for Jesus’ Sonship of God by offering as the proof of it Jesus’ resurrection.