How was the Davidic royal line preserved until Jesus' birth?
God's promise to David ensured a royal family, the house of David. Despite ungodly kings, the royal blood line was kept intact until Christ's birth, coming especially through Mary as physical mother, while Joseph served as legal father included in the Davidic genealogy.
Les explains that 2 Samuel 7 records God giving David the promise that through him would come a royal family, the house of David. Even though ungodly kings came down through history, the royal blood kept together until the birth of Christ. This line was preserved especially through Mary as the physical mother; God is the Father, but Joseph was the legal father and so had to be included in the Davidic genealogy.