10 “But You, Lord, art the Most Highest for evermore”. Waiting above in Your eternity until the season of the wicked be past, and that of the just come. “For lo.” Listen, brethren. Already he who speaks (for he speaks in our person, in the person of Christ's body, for Christ speaks in His own body, that is, in His Church), has joined himself unto the eternity of God: as I a little before was saying unto you, God is long-suffering and patient, and allows all those evil deeds which He sees to be done by wicked men. Wherefore? Because He is eternal, and sees what He keeps for them. Do you also wish to be long-suffering and patient? Join yourself to the eternity of God: together with Him wait for those things which are beneath you: for when your heart shall have cleaved unto the Most Highest, all mortal things will be beneath you: say then what follows, “For lo, your enemies shall perish.” Those who now flourish, shall afterwards perish. Who are the enemies of God? Brethren, perhaps ye think those only enemies of God who blaspheme? They indeed are so, and those wicked men who neither in tongue nor in thought cease to injure God. And what do they do to the eternal, most high God? If you strike with your fist upon a pillar, you are hurt: and do you think that where you strike God with your blasphemy, you are not yourself broken? For you do nothing to God. But the enemies of God are openly blasphemers, and daily they are found hidden. Beware of such enmities of God. For the Scripture reveals some such secret enemies of God: that because you know them not in your heart, you may know in God's Scriptures, and beware of being found with them. James says openly in his Epistle, “Do you not know that the friendship of the world is enmity with God?” You have heard. Do you wish not to be an enemy of God? Be not a friend of this world: for if you are a friend of this world, you will be an enemy of God. For as a wife cannot be an adulteress, unless she be an enemy to her own husband: so a soul which is an adulteress through its love of worldly things, cannot but be an enemy to God. It fears, but loves not: it fears punishment but is not delighted with righteousness. All lovers of the world, therefore, are enemies of God, all the curious after trifles, all consulters of diviners astrologers, and evil spirits. Let them enter, or not enter, Churches: they are enemies of God. They may flourish for a season like grass, but they will perish, when He begins to visit them, and pronounce His sentence upon all flesh. Join yourself to the Scripture of God, and say with this Psalm, “For lo, your enemies shall perish”. Be not found there, where they shall perish. “And all the workers of iniquity shall be destroyed.”
11....“But mine horn shall be exalted like the horn of an unicorn”. Why did He say, “like the horn of an unicorn”? Sometimes an unicorn signifies pride, sometimes it means the lifting up of unity; because unity is lifted up, all heresies shall perish with the enemies of God. And “mine horn shall be exalted like an unicorn.” When will it be so? “And mine old age shall be in the fatness of mercy.” Why did he say, “my old age”? He means, my last days; as our old age is the last season in our lives, so the whole of what the body of Christ at present suffers in labours, in cares, in watchings, in hunger, in thirst, in stumbling-blocks, in wickednesses, in tribulations, is its youth: its old age, that is, its last days, will be in joy. And beware, beloved, that you think not death meant also, in that he has spoken of old age: for man grows old in the flesh for this reason, that he may die. The old age of the Church will be white with good works, but it shall not decay through death. What the head of the old man is, that our works will be. You see how the head grows old, and whitens, as fast as old age approaches. Thou sometimes dost seek in the head of one who grows old duly in his own course a black hair, yet you find it not: thus when our life shall have been such, that the blackness of sins may be sought, and none found, that old age is youthful, is green, and ever will be green. You have heard of the grass of sinners, hear ye of the old age of the righteous: “My old age shall be in the fathers of mercy.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)