18 Time and the lapse of ages make no difference to a Spirit. Christ is one and the same Christ, whether in the body, or abiding by the Spirit in the prophets. Speaking through the mouth of the holy Patriarch David, He says, Your God, O God, has anointed You with the oil of gladness above Your fellows, which refers to no less a mystery than the Dispensation of His assumption of flesh. He, Who now sends the message to His brethren that their Father is His Father, and their God His God, announced Himself then as anointed by His God above His fellows.
No one is fellow to the Only-begotten Christ, God the Word: but we know that we are His fellows by the assumption which made Him flesh. That anointing did not exalt the blessed and incorruptible Begotten Who abides in the nature of God, but it established the mystery of His body, and sanctified the manhood which He assumed. To this the Apostle Peter witnesses, Of a truth in this city were they gathered together against Your holy Son Jesus, Whom You anointed: and on another occasion, You know that the saying was published through all Judæa, beginning from Galilee, after the baptism which John preached: even Jesus of Nazareth, how that God anointed Him with the Holy Ghost and with power. Jesus was anointed, therefore, that the mystery of the regeneration of flesh might be accomplished.
Nor are we left in doubt how He was thus anointed with the Spirit of God and with power, when we listen to the Father's voice, as it spoke when He came up out of the Jordan, You are My Son, this day have I begotten You. Thus is testified the sanctification of His flesh, and in this testimony we must recognise His anointing with the power of the Spirit.
Source: On the Trinity (New Advent)