17 “Therefore, God, Your God, has anointed You.” It was for this reason that He anointed you, that you might love righteousness, and hate iniquity. And observe in what way he expresses himself. “Therefore, God, Your God, has anointed You:” i.e. “God has anointed You, O God.” “God” is “anointed” by God. For in the Latin it is thought to be the same case of the noun repeated: in the Greek however there is a most evident distinction; one being the name of the Person addressed; and one His who makes the address, saying, “God has anointed You.”
“O God, Your God has anointed You,” just as if He were saying, “Therefore has Your God, O God, anointed You.” Take it in that sense, understand it in that sense; that such is the sense is most evident in the Greek. Who then is the God that is “anointed” by God? Let the Jews tell us; these Scriptures are common to us and them. It was God, who was anointed by God: you hear of an “Anointed” one; understand it to mean “Christ.” For the name of “Christ” comes from “chrism;” this name by which He is called “Christ” expresses “unction:” nor were kings and prophets anointed in any kingdom, in any other place, save in that kingdom where Christ was prophesied of, where He was anointed, and from whence the Name of Christ was to come.
It is found nowhere else at all: in no one nation or kingdom. God, then, was anointed by God; with what oil was He anointed, but a spiritual one? For the visible oil is in the sign, the invisible oil is in the mystery; the spiritual oil is within. “God” then was “anointed” for us, and sent unto us; and God Himself was man, in order that He might be “anointed:” but He was man in such a way as to be God still. He was God in such a way as not to disdain to be man. “Very man and very God;” in nothing deceitful, in nothing false, as being everywhere true, everywhere “the Truth” itself. God then is man; and it was for this cause that “God” was “anointed,” because God was Man, and became “Christ.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)