When David proposed to build God a house, God answered through the prophet Nathan that He would instead build David a house: 'I will raise up thy seed after thee... and I will establish the throne of his kingdom FOR EVER. I will be to him a father, and he shall be to me a son' (2 Samuel 7:12–14). No merely political dynasty could carry that word 'for ever' — and the psalms and prophets kept the oath alive through exile and ruin (Psalm 89; Isaiah 9:7; Jeremiah 23:5).
The angel Gabriel's annunciation to Mary is a deliberate, clause-by-clause echo of Nathan's oracle: '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 David his father; and he shall reign in the house of Jacob for ever. And of his kingdom there shall be no end' (Luke 1:32–33) — words the Church fixed into the Nicene Creed: cuius regni non erit finis.
Both Gospel genealogies route the legal and natural descent of Jesus through David (Matthew 1:6; Luke 3:31); 'Son of David' is the title the blind and the crowds cry out (Matthew 9:27; 21:9); and the Resurrection is preached by Peter as the enthronement the oath intended — David 'foreseeing this, spoke of the resurrection of Christ' (Acts 2:30–31).