By What Rule in the Scriptures It is Understood that the Son is Now Equal and Now Less
22 Wherefore, having mastered this rule for interpreting the Scriptures concerning the Son of God, that we are to distinguish in them what relates to the form of God, in which He is equal to the Father, and what to the form of a servant which He took, in which He is less than the Father; we shall not be disquieted by apparently contrary and mutually repugnant sayings of the sacred books. For both the Son and the Holy Spirit, according to the form of God, are equal to the Father, because neither of them is a creature, as we have already shown: but according to the form of a servant He is less than the Father, because He Himself has said, “My Father is greater than I;” and He is less than Himself, because it is said of Him, He emptied Himself; and He is less than the Holy Spirit, because He Himself says, “Whosoever speaks a word against the Son of man, it shall be forgiven him; but whosoever speaks against the Holy Ghost, it shall not be forgiven Him.” And in the Spirit too He wrought miracles, saying: “But if I with the Spirit of God cast out devils, no doubt the kingdom of God has come upon you.” And in Isaiah He says—in the lesson which He Himself read in the synagogue, and showed without a scruple of doubt to be fulfilled concerning Himself—“The Spirit of the Lord God,” He says, “is upon me: because He has anointed me to preach good tidings unto the meek He has sent me to proclaim liberty to the captives,” etc.: for the doing of which things He therefore declares Himself to be “sent,” because the Spirit of God is upon Him. According to the form of God, all things were made by Him; according to the form of a servant, He was Himself made of a woman, made under the law. According to the form of God, He and the Father are one; according to the form of a servant, He came not to do His own will, but the will of Him that sent Him. According to the form of God, “As the Father has life in Himself, so has He given to the Son to have life in Himself;” according to the form of a servant, His “soul is sorrowful even unto death;” and, “O my Father,” He says, “if it be possible, let this cup pass from me.” According to the form of God, “He is the True God, and eternal life;” according to the form of a servant, “He became obedient unto death, even the death of the cross.” — 23. According to the form of God, all things that the Father has are His, and “All mine,” He says, “are Yours, and Yours are mine;” according to the form of a servant, the doctrine is not His own, but His that sent Him.
Source: On the Holy Trinity (New Advent)