The LORD said unto my Lord, Sit thou at my right hand, until I make thine enemies thy footstool.
The Lord — God the father.
Said — Decreed it from eternity, and in due time published this decree, and actually executed it; which he did when he raised up Christ from the dead, and brought him into his heavenly mansion.
Unto — Unto his son the Messiah, whom David designedly calls his Lord, to admonish the whole church, that although he was his son according to his human nature, yet he had an higher nature, and was also his Lord, as being God blessed for ever, and consequently Lord of all things. The Hebrew word Adon is one of God's titles, signifying his power and authority over all things, and therefore is most fitly given to the Messiah, to whom God hath delegated all his power Matthew 28:18.
Sit — Now take thy rest and the possession of that sovereign kingdom and glory, which by right belongeth to thee; do thou rule with me with equal power and majesty, as thou art God; and with an authority and honour far above all creatures, as thou art man.
Make — By my almighty power communicated to thee as God by eternal generation, and vouchsafed to thee as mediator.
Enemies — All ungodly men, sin and death, and the devil.
Footstool — Thy slaves and vassals.
 The LORD shall send the rod of thy strength out of Zion: rule thou in the midst of thine enemies.
The rod — Thy strong or powerful rod, and the rod is put for his scepter, or kingly power: but as the kingdom of Christ is not carnal, but spiritual, so this scepter is nothing else but his word.
Zion — From Jerusalem.
 Thy people shall be willing in the day of thy power, in the beauties of holiness from the womb of the morning: thou hast the dew of thy youth.
People — Thy subjects, shall offer thee as their king and Lord, not oxen or sheep, but themselves, their souls and bodies, as living sacrifices, and as freewill-offerings, giving up themselves to the Lord, 2 Corinthians 8:5, to live to him, and to die for him.
The day — When thou shalt take into thy hands the rod of thy strength, and set up thy kingdom in the world.
In the beauties — Adorned with the beautiful and glorious robes of righteousness and true holiness.
The dew — That is, thy offspring (the members of the Christian church) shall be more numerous than the drops of the morning dew.
 The LORD hath sworn, and will not repent, Thou art a priest for ever after the order of Melchizedek.
Sworn — That this priesthood might be made sure and irrevocable.
 The Lord at thy right hand shall strike through kings in the day of his wrath.
The Lord — God the son; the Lord, who is at thy right-hand.
Strike — Shall destroy all those kings who are obstinate enemies to him.
 He shall judge among the heathen, he shall fill the places with the dead bodies; he shall wound the heads over many countries.
Judge — Condemn and punish them.
The places — Or, the place of battle.
Dead bodies — Of his enemies.
Heads — All those princes who oppose him. But this and the like passages are not to be understood grossly, but spiritually, according to the nature of Christ's kingdom.
 He shall drink of the brook in the way: therefore shall he lift up the head.
Drink — He shall have a large portion of afflictions, while he is in the way or course of his life, before he comes to that honour of sitting at his father's right-hand. Waters in scripture frequently signify sufferings. To drink of them, signifies to feel or bear them.
Therefore — He shall be exalted to great glory and felicity.