From the additional notes to Charles Spurgeon's The Treasury of David, Psalm 110:
Verse 1. Make thine enemies thy footstool! This expression, that the conquest of Christ's enemies shall be but as the removing of a stool into its place, notes unto us two things: first, the easiness of God's victory over the enemies of Christ. They are before him as nothing, less than nothing, the drop of a bucket, the dust of the balance, a very little thing...Secondly, as this putting of Christ's enemies like a stool under the feet notes easiness, so also it notes order or beauty too. When Christ's enemies shall be under his foot, then there shall be a right order in things; then it shall indeed appear that God is a God of order, and therefore the day wherein that shall be done, is called "the times of the restitution of all things, "Ac 3:21.
The putting of Christ's enemies under his feet is an act of justice; and of all others, justice is the most orderly virtue, that which keepeth beauty upon the face of a people, as consisting itself in symmetry and proportion. This putting of Christ's enemies as a stool under his feet, also denotes unto us two things in reference to Christ: first, his rest, and secondly, his triumph. To stand, in the Scripture phrase, denotes ministry, and to sit, rest; and there is no posture so easy as to sit with a stool under one's feet. Till Christ's enemies then be all under his feet, he is not fully in his rest.
Furthermore, this "footstool" under Christ's feet, in reference to his enemies, denotes unto us four things. First, the extreme shame and confusion which they shall everlastingly suffer, the utter abasing and bringing down of all that exalteth itself against Christ. Secondly, hereby is noted the burden which wicked men must bear: the footstool beareth the weight of the body, so must the enemies of Christ bear the weight of his heavy and everlasting wrath upon their souls. Thirdly, herein is noted the relation which the just recompense of God bears unto the sins of ungodly men. Thus will Christ deal with his enemies at the last day. Here they trample upon Christ in his word, in his ways, in his members; they make the saints bow down for them to go over, and make them as the pavements on the ground; they tread under foot the blood of the covenant, and the sanctuary of the Lord, and put Christ to shame; but there their own measure shall be returned into their bosoms, they shall be constrained to confess as Adonibezek, "As I have done, so God hath requited me."
Lastly, herein we may note the great power and wisdom of Christ in turning the malice and mischief of his enemies unto his own use and advantage; and so ordering wicked men that though they intend nothing but extirpation and ruin to his kingdom, yet they shall be useful unto him, and, against their own wills, serviceable to those glorious ends, in the accomplishing whereof he shall be admired by all those that believe. As in a great house there is necessary use of vessels of dishonour, destined unto sordid and mean, but yet daily, services: so in the great house of God, wicked men are his utensils and household instruments, as footstools and staves, and vessels wherein there is no pleasure, though of them there may be good use.
Condensed from Edward Reynolds.