Augustine. Exposition on the Book of Psalms. Schaff, Philip (editor). New York: Christian Literature Publishing Co, 1886. Nicene and Post-Nicene Church Fathers: Series 1: Volume VIII. Re-published 2014, Catholic Way Publishing. Kindle electronic edition, ISBN-13: 978-1-78379-372-3.
From the outset of his comments on Psalm 110, Augustine is plain that he considers it a prophecy of Jesus (Augustine Psalms, loc. 338251, par. 1). Jesus himself referred to the first verse, when skeptics discussed with him his claims to be both Son of David and Lord. Verse one depicts him as the one who sits at the Father's right hand, placed overall his enemies (Augustine Psalms, loc. 338256, par. 2). Matthew's Gospel refers to Christ as the Son of David. By faith he is recognized as Lord. He is further recognized as the one who sits at the right hand of the Father (Augustine Psalms, loc. 338266, par. 3). Augustine points out that it should be no great surprise to find this true. If an earthly king is raised up and is not the son of the former king, he naturally becomes his father's lord. People often end up in positions of authority over their parents (Augustine Psalms, loc. 338271, par. 3). Augustine describes the process of the exaltation of Christ over his enemies as a matter of degrees. Those hostile to Christ are gradually placed in subjection to him.
In verse two, then, God sends a rod of power from Zion, ruling over the nations (Augustine Psalms, loc. 338286, par. 5). Augustine sees this as a move which has been accomplished already. Jesus has ruled from the beginning according to verse three, since he was the Word from the beginning (Augustine Psalms, loc. 338306, par. 8). This, then, is also an affirmation of at least two persons of the Godhead, ruling from eternity.
Somehow, then, mystically, God the Son is begotten, from the beginning, "before the morning star" (Augustine Psalms, loc. 338316, par. 10). The language is strongly anthropomorphic. Yet Christians do not confess any sort of god created in man's image. The Son is eternally pre-existent, with physicality to arise later.
Verse four goes on to speak in very physical terms, about Melchizedek (Augustine Psalms, loc. 338331, par. 11). God has made this clear, through his Word which can never fail. He alone can swear an oath reliably.
Augustine observes that in verse five the Lord is on the right hand of the Father, "as if they changed seats" (Augustine Psalms, loc. 338346, par. 12). From this seat, the Lord judges the nations (v. 6). Augustine sees this as the ruin of the unbeliever. All who exalt themselves against God will be humbled (Augustine Psalms, loc. 338361, par. 13).