2 “But the Comfort,” He adds, “[which is] the Holy Ghost, whom the Father will send in my name, He shall teach you all things, and bring all things to your remembrance, whatsoever I have said unto you.” Is it, then, that the Son speaks, and the Holy Spirit teaches, so that we merely get hold of the words that are uttered by the Son, and then understand them by the teaching of the Spirit as if the Son could speak without the Holy Spirit, or the Holy Spirit teach without the Son: or is it not rather that the Son also teaches and the Spirit speaks, and, when it is God that speaks and teaches anything, that the Trinity itself is speaking and teaching?
And just because it is a Trinity, its persons required to be introduced individually, so that we might hear it in its distinct personality, and understand its inseparable nature. Listen to the Father speaking in the passage where you read, “The Lord said unto me, You are my Son:” listen to Him also teaching, in that where you read, “Every man that has heard, and has learned of the Father, comes unto me.” The Son, on the other hand, you have just heard speaking; for He says of Himself, “Whatsoever I have said unto you:” and if you would also know Him as a Teacher, bethink yourself of the Master, when He says, “One is your Master, even Christ.” Furthermore, of the Holy Spirit, whom you have just been told of as a Teacher in the words, “He shall teach you all things,” listen to Him also speaking, where you read in the Acts of the Apostles, that the Holy Spirit said to the blessed Peter, “Go with them, for I have sent them.” The whole Trinity, therefore, both speaks and teaches: but were it not also brought before us in its individual personality, it would certainly altogether surpass the power of human weakness to comprehend it. For as it is altogether inseparable in itself, it could never be known as the Trinity, were it always spoken of inseparably; for when we speak of the Father, and the Son, and the Holy Spirit, we certainly do not pronounce them simultaneously, and yet in themselves they cannot be else than simultaneous. But when He added, “He will bring to your remembrance,” we ought also to understand that we are commanded not to forget that these pre-eminently salutary admonitions are part of that grace which the Holy Spirit brings to our remembrance.
Source: Tractates on the Gospel of John (New Advent)