18 “And my mouth has spoken in my tribulation.” How sweet ofttimes is tribulation, how necessary! In that case what has the mouth of the same spoken in his tribulation? “Holocausts marrowed I will offer to You”. What is “marrowed”? Within may I keep Your love, it shall not be on the surface, in my marrow it shall be that I love You. For there is nothing more inward than our marrow: the bones are more inward than the flesh, the marrow is more inward than those same bones.
Whosoever therefore on the surface loves God, desires rather to please men, but having some other affection within, he offers not holocausts of marrow: but into whosesoever marrow He looks, him He receives whole. “With incense and rams.” The rams are the rulers of the Church: the whole Body of Christ is speaking: this is the thing which he offers to God. Incense is what? Prayer. “With incense and rams.” For especially the rams do pray for the flocks. “I will offer to You oxen with he-goats.”
Oxen we find treading out grain, and the same are offered to God. The Apostle has said, that of the preachers of the Gospel must be understood that which has been written, “Of the ox treading out grain the mouth you shall not muzzle. Does God care for oxen?” Therefore great are those rams, great the oxen. What of the rest, that perchance are conscious of certain sins, that perchance in the very road have slipped, and, having been wounded, by penitence are being healed? Shall they too continue, and to the holocausts shall they not belong?
Let them not fear, he has added he-goats also. “I will offer to You oxen with he-goats.” By the very yoking are saved the he-goats; of themselves they have no strength, being yoked to bulls they are accepted. For they have made friends of the mammon of iniquity, that the same may receive them into everlasting tabernacles. Therefore those he-goats shall not be on the left, because they have made to themselves friends of the mammon of iniquity. But what he-goats shall be on the left? They to whom shall be said, “I hungred, and you gave me not to eat:” not they that have redeemed their sins by almsdeeds.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)