Psalm 110: David’s Thoughts About Melchizedek
Psalm 110 is the first mention in the Bible, that the coming Messiah would be a priest. A king, yes, that is foretold early in the OT. But it was not until David, meditating on the significance of Melchizedek in Genesis, that the OT revealed the Messiah-priest.
We don’t know the circumstances of how or when David penned Psalm 110, but we can speculate on that, from the events in David’s life. Very possibly, David thought of Melchizedek and his significance, when he conquered and took Jerusalem. Jerusalem, the city of peace, was the Salem of old. Melchizedek was part of that ancient Jerusalem. Besides that event, though, David likely thought more about this after his sin with Bathsheba, a time when he considered his own great need for an eternal high priest — something beyond the Aaronic priesthood.
What we learn from study of the Old Testament (as well as the New), is that the writers of inspired scripture were themselves great students of the Word, of the parts they had access to in their day. Isaiah for instance relies heavily on the Pentateuch, and especially on Deuteronomy 31-32. Zechariah referred to Isaiah’s prophecy, a book he was clearly familiar with. Here, too, with the case of Psalm 110 we see David as a student of the Word.