City of God 481How Like the Prophecy About Christ in the 89th Psalm is to the Things Promised in Nathan’s Prophecy in the Books of Samuel.

Wherefore also in the 89th Psalm, of which the title is, "An instruction for himself by Ethan the Israelite," mention is made of the promises God made to king David, and some things are there added similar to those found in the Book of Samuel, such as this, "I have sworn to David my servant that I will prepare his seed for ever." And again, "Then thou spakest in vision to thy sons, and saidst, I have laid help upon the mighty One, and have exalted the chosen One out of my people. I have found David my servant, and with my holy oil I have anointed him. For mine hand shall help him, and mine arm shall strengthen him. The enemy shall not prevail against him, and the son of iniquity shall harm him no more. And I will beat down his foes from before his face, and those that hate him will I put to flight. And my truth and my mercy shall be with him, and in my name shall his horn be exalted. I will set his hand also in the sea, and his right hand in the rivers. He shall cry unto me, Thou art my Father, my God, and the undertaker of my salvation. Also I will make him my first-born, high among the kings of the earth. My mercy will I keep for him for evermore, and my covenant shall be faithful (sure) with him. His seed also will I set for ever and ever, and his throne as the days of heaven." Which words, when rightly understood, are all understood to be about the Lord Jesus Christ, under the name of David, on account of the form of a servant, which the same Mediator assumed from the virgin of the seed of David. For immediately something is said about the sins of his children, such as is set down in the Book of Samuel, and is more readily taken as if of Solomon. For there, that is, in the Book of Samuel, he says, "And if he commit iniquity I will chasten him with the rod of men, and with the stripes of the sons of men; but my mercy will I not take away from him," meaning by stripes the strokes of correction. Hence that saying, "Touch ye not my christs." For what else is that than, Do not harm them? But in the psalm, when speaking as if of David, He says something of the same kind there too. "If his children," saith He, "forsake my law, and walk not in my judgments; if they profane my righteousnesses, and keep not my commandments; I will visit their iniquities with the rod, and their faults with stripes: but my mercy I will not make void from him." He did not say "from them," although He spoke of his children, not of himself; but he said "from him," which means the same thing if rightly understood. For of Christ Himself, who is the head of the Church, there could not be found any sins which required to be divinely restrained by human correction, mercy being still continued; but they are found in His body and members, which is His people. Therefore in the Book of Samuel it is said, "iniquity of Him," but in the psalm, "of His children," that we may understand that what is said of His body is in some way said of Himself. Wherefore also, when Saul persecuted His body, that is, His believing people, He Himself saith from heaven, "Saul, Saul, why persecutest thou me?" Then in the following words of the psalm He says, "Neither will I hurt in my truth, nor profane my covenant, and the things that proceed from my lips I will not disallow. Once have I sworn by my holiness, if I lie unto David,"--that is, I will in no wise lie unto David; for Scripture is wont to speak thus. But what that is in which He will not lie, He adds, saying, "His seed shall endure for ever, and his throne as the sun before me, and as the moon perfected for ever, and a faithful witness in heaven."