Verses 1-7
Psalm 110 The ideal priest-king
Melchizedek was priest-king of the Canaanite city-state of Salem, later known as Jerusalem. He was a man so pure and upright that he was called king of peace, king of righteousness and priest of the Most High God (Hebrews 7:1-10). He first appears in the biblical record when he met and blessed Abraham, who was returning after a victory over some raiders. Abraham refused to take any reward from those who benefited from the victory, but instead made offerings to God's priest. In this way he acknow ledged that God was sovereign ruler in human affairs (Genesis 14:1-24).
When David conquered Jerusalem and set up his throne there, he became heir to Melchizedek as Jerusalem's ideal priest-king. (The Melchizedek kind of priesthood was distinct from the Aaronic kind of priesthood.) As God's representative, David also was to be king of peace, king of righteousness and priest of the Most High God. It seems that David wrote this psalm to be sung by the temple singers to celebrate the establishment of his throne in Jerusalem (2 Samuel 5:6-12).
To David it seems that, with his conquest of Jerusalem, God has given him victory over all his enemies and invited him to sit in the place of supreme power (1). From Mount Zion in Jerusalem, David rules his people and conquers his foes (2). The people willingly offer themselves to the king for his service in spreading his rule throughout the land. An army of young men with the life-giving freshness of dew and the strength of youth present themselves to the king (3). Just as the authority of Melchizedek, God's representative, had no historical or national limits, so David's authority in the name of God is limitless, in time and extent (4). God will lead his king to universal conquest and rule (5-6). He will refresh him by renewing his vigour continually, till he stands victorious, master of all (7).
It becomes clear as we read the psalm that David was but a very faint picture of the universal priest-king. Jews in later times interpreted the psalm as applying to the Messiah, and Jesus agreed that this was a correct application (Matthew 22:42-45; see also Matthew 26:64; Acts 2:34-35; Hebrews 1:3; Hebrews 1:13). The Melchizedek priesthood that David inherited was a priesthood in name only, a mere title to add to the other titles held by the Israelite king. Jesus Christ's priesthood after the order of Melchizedek is complete and never-ending (Hebrews 5:6; Hebrews 7:1-28). His final conquest and royal rule will be universal (1 Corinthians 15:24-25; Revelation 19:11-21; Revelation 20:1-6).