What is the whole idea of Christ's first advent?
The whole idea of Christ's first advent was that He came to be the promised King according to the covenants. He made His appearance to the nation of Israel as the last possible son of David who could be the King, fulfilling God's promises to the Israelites.
Les explains that Christ's first advent was for the nation of Israel, to present Himself as the promised King according to the covenants. The genealogies show Joseph and Mary, and Christ is the last possible son of David that could be the King. Paul confirms this in verse 4, speaking of Israelites, to whom pertain the adoption, glory, covenants, giving of the law, service of God, and promises. Les emphasizes God promised the nation over and over, and that is the whole idea of His first advent.