Augustine, On the Holy Trinity
By what Rule in the Scriptures it is Understood that the Son is Now Equal and Now Less.
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 speaketh a word against the Son of man, it shall be forgiven him; but whosoever speaketh 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 is 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 hath anointed me to preach good tidings unto the meek He hath 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 hath life in Himself, so hath 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 hath are His,  and “All mine,” He says, “are Thine, and Thine are mine;”  according to the form of a servant, the doctrine is not His own, but His that sent Him. 
Footnotes:  John 14:28,  Philippians 2:7,  Matthew 12:32,  Matthew 12:28,  Isaiah 61:1; Luke 4:18, 19,  John 1:3,  Galatians 4:4,  John 10:30,  John 6:38,  John 5:26,  Matthew 26:38, 39,  1 John 5:20,  Philippians 2:8,  John 17:15,  John 17:10,  John 7:16