Through which line did God promise the Messiah would come?
God promised that the Messiah, the King, would come through the line of David, specifically descending through Solomon and Nathan. This covenant promise was secure, not dependent on Israel's actions. The Abrahamic seed also points to Christ, but the Davidic line is the specific royal lineage for the Messiah.
Les explains that God's covenant with David promised that through David's line, coming down through Solomon and Nathan, the King, the Messiah, would one day come. This had to happen regardless of what Israel did, because the covenant stands secure; God will never go back on His Word. Les also notes that the promises to Abraham and his seed pointed to Christ, the singular seed, as Paul argues in Galatians 3.