19 But the Word was God, and with God in the beginning, and therefore the anointing could neither be related nor explained, if it referred to that nature, of which we are told nothing, except that it was in the beginning. And in fact He Who was God had no need to anoint Himself with the Spirit and power of God, when He was Himself the Spirit and power of God. So He, being God, was anointed by His God above His fellows. And, although there were many Christs (i.e. anointed persons) according to the Law before the Dispensation of the flesh, yet Christ, Who was anointed above His fellows, came after them, for He was preferred above His anointed fellows.
Accordingly, the words of the prophecy bring out the fact that the anointing took place in time, and comparatively late in time. You have loved righteousness and hated iniquity: therefore Your God, O God, has anointed You with the oil of gladness above Your fellows. Now, a fact which follows later upon other facts, cannot be dated before them. That a reward be deserved postulates as a prior condition the existence of one who can deserve it, for merit earned implies that there has been one capable of acquiring it.
If, therefore, we attribute the birth of the Only-begotten God to this anointing, which is His reward for loving righteousness and hating iniquity, we shall be regarding Him not as born, but as promoted by unction, to be the Only-begotten God. But then we imply that He advanced with gradual progress and promotion to perfect divinity, and that He was not born God, but afterwards for His merit anointed God. Thus we shall make Christ as God Himself conditioned, whereas He is the final cause of all conditions; and what becomes then of the Apostle's words, All things are through Him and in Him, and He is before all, and in Him all things consist ? The Lord Jesus Christ was not deified because of anything, or by means of anything, but was born God: God by origin, not promoted to divinity for any cause after His birth, but as the Son; and one in kind with God because begotten of Him.
His anointing then, though it is the result of a cause, did not enhance that in Him, which could not be made more perfect. It concerned that part of Him which was to be made perfect through the perfection of the Mystery: that is, our manhood was sanctified in Christ by unction. If then the prophet here also teaches us the dispensation of the servant, for which Christ is anointed by His God above His fellows, and that because He loved righteousness and hated iniquity, then surely the words of the prophet must refer to that nature in Christ, by which He has fellows through His assumption of flesh.
Can we doubt this when we note how carefully the Spirit of prophecy chooses His words? God is anointed by His God; that is, in His own nature He is God, but in the dispensation of the anointing God is His God. God is anointed: but tell me, is that Word anointed, Who was God in the beginning? Manifestly not, for the anointing comes after His divine birth. It was then not the begotten Word, God with God in the beginning, Who was anointed, but that nature in God which came to Him through the dispensation later than His divinity: and when His God anointed Him, He anointed in Him the whole nature of the servant, which He assumed in the mystery of His flesh.
Source: On the Trinity (New Advent)