God had made an unbreakable oath and covenant with David. When the exile seemed to end the line of kings from David's family (Psalm 89:38), it created a crisis for God’s people. The placement of Psalm 110 in Book Five points again to the future of David’s line. David’s descendant must live according to God’s standard of integrity (Psalm 101). His rule will also be shaped by God’s kingship (Psalms 93–100).
You are a priest forever: The role of God’s messianic ruler changes. The kings from David’s family had a limited priestly role. They supported and organized the worship of the temple. For example, David brought the ark to Jerusalem (2 Samuel 6:15) and organized temple services (1 Chronicles 6:31–48; 15:11–26; 16:4–42; 23:4–32; 25:1). Solomon oversaw the construction of the temple (1 Kings 5:1–7:51).
Even so, the offices of king and priest were separate in Israel. In the messianic kingdom, the king will also serve as a priest. He will be like Melchizedek, the king of Salem who was also a priest (Genesis 14:18). The New Testament explains that Jesus Christ fulfills this new order of priesthood (see Hebrews 5:6–10; 6:20–7:25).