Psalm 110 is quoted frequently in the New Testament. Jesus used it to correct the wrong notions of the Pharisees regarding the Son of David (the Messiah). Peter quoted it when preaching to the Jews on the Day of Pentecost and Paul referred to it when describing what happened to Jesus after his ascension to heaven. Several commentators have suggested that the Book of Hebrews is a sermon based on this psalm (both Hebrews and this psalm depict Jesus as a priest like Melchizedek). The apostle John cited from this psalm in the Book of Revelation.
The psalm is a Messianic psalm in the fullest meaning of that description. Some psalms are called Messianic only because they illustrate what Jesus would do (Psalm 72 is an example). Other psalms are Messianic because part of them are prophecies of Jesus (Psalms 40 and 118 are examples). The third type of Messianic psalm are those psalms that are only about Jesus (Psalms 2 and 22 are example, as is Psalm 110).
David begins with the joyous description of the Messiah's entrance to heaven and enthronement of God's throne. David refers to two divine persons in his opening line: 'The Lord (the Father) said to my Lord (the Son).' We should note how David, although he is a king, regards himself as the subject of King Jesus, and does so long before he became a man.
In addition to being a king, David was also a prophet and here he predicts with God-given insight the reign of King Jesus between his ascension and his ultimate triumph when he returns to judge the world. What will his reign be like?
First, it will be a reign with guaranteed success (v. 1). The heavenly Father gladly promises his Son that all his enemies will become his footstool (a picture of total triumph). Jesus cannot be defeated or removed from his throne.
We should also see in the Father's proclamation his great delight in what his Son achieved on earth. The Father's invitation to sit at his right hand is the fulfilment of the agreement made between the Father and the Son before the universe was created. Basically the agreement involved the Son coming to earth in order to pay the penalty of sin. On doing so, he would be highly exalted by the Father. And here is David rejoicing to describe it.
Second, it will be a reign marked by sovereign power (v. 2). What is the sceptre that David refers to in verse 2? Clearly, the sceptre belongs to the Son (your sceptre), yet is also sent out by the Father. The sceptre is a picture of the Holy Spirit, and by it is by the Spirit that Jesus works powerfully and graciously in the hearts of his enemies through the gospel.
Third, Jesus will have a large army of godly sinners (v. 3). Throughout the day of his power (the phrase does not refer to an individual day, but to the entire period when he will reign), many will volunteer to serve him. This aspect will begin at the onset of this period (the womb of the morning), which is probably a reference to the Day of Pentecost. It is important to note this army is not made up of conscripts who are forced reluctantly to serve. Instead they freely respond to his invitation to enlist. Of course, while they are not conscripts, it is true that all of them once belonged to the army of the enemy.
Fourth, this army fights in an unusual way (v. 3). Each of them is given the garments of holiness to wear. This is how they overcome everything and everyone that attempts to hinder their advance. Holiness is a vast subject. It includes dedication to Jesus, diligent delight in obeying God's law, and determination not to stain their garments. The picture that David uses is of an army of priests advancing down the centuries and increasing in number as they sing the praises of God.
Fifth, the King never grows old (v. 3). David, whenever he wrote the psalm, would have been fully aware of the fact that his abilities diminished as he became an old man. In contrast, it will never be said of the Messiah that his powers will decrease. Jesus the King possesses eternal youth for ever. He will never have weakness or frustration in all that he does. Beholding this youthful King will be very refreshing, just like experiencing the effects of dew.