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Acts 2:30 Therefore being a prophet, and knowing that God had sworn with an oath to him, that of the fruit of his loins, according to the flesh, he would raise up Christ to sit on his throne;
Matthew 1:18 Now the birth of Jesus Christ was on this wise: When as his mother Mary was espoused to Joseph, before they came together, she was found with child of the Holy Ghost.
I have seen it asked: How can Jesus be descended from David if Mary and the Holy Ghost were involved, and not Joseph?
We refer our reader to Glenn Miller's exposition on the genealogies of Jesus. The reconciliation of the genealogies - attributing one to Joseph and the other to Mary - comes into play here. The "he" in Acts 2:30 is David, and the promise is that one of David's descendants would be the Christ. This is fulfilled biologically through Mary, who was descended from David.