Another Comforter
John 14-16, the promise of the Spirit, as the early church read it
In the upper room Jesus promises "another Comforter," the Spirit of truth, who will abide forever, teach all things, and guide into all truth. The early church drew from these verses both its confidence in the Spirit's full deity, and, eventually, its longest-running disagreement, over how the Spirit "proceeds." The Fathers below hold the first in common and divide on the second, which is itself worth seeing plainly. With a plain restatement throughout.
Each Father's words are verbatim and attributed (Catena Aurea, public domain, lightly corrected for scan errors). The box marked "In plain terms" is our own restatement, never the Father's words.
John 14:16-17 · KJVAnd I will pray the Father, and he shall give you another Comforter, that he may abide with you for ever; even the Spirit of truth…
"The Holy Ghost was another Comforter: differing not in nature, but in operation."
Didymus (in Alcuin)"'That He may abide with you for ever.' The Spirit does not depart even at death… the Holy Ghost will not suffer death, or go away, as He has done. But that the mention of the Comforter might not lead them to expect another incarnation… He adds, 'Even the Spirit of truth Whom the world cannot receive.'"
St. John Chrysostom"Another Comforter" is the key. The Spirit is another alongside Christ, a distinct Person, and yet "differing not in nature", of the same Godhead, not a lesser power. Unlike Christ in the flesh, He will not die or depart but "abide forever." He is not a second incarnation to be seen with the eye, but the Spirit of truth dwelling within. The early church reads here the full deity and permanence of the Holy Spirit.
John 15:26 · KJVBut when the Comforter is come, whom I will send unto you from the Father, even the Spirit of truth, which proceedeth from the Father, he shall testify of me:
"Elsewhere He says that the Father sends the Spirit; now He says He does: 'Whom I will send to you,' thus declaring the equality of the Father and the Son. That He might not be thought, however… another and rival source of the Spirit, He adds, 'From the Father,' i.e., the Father agreeing, and taking an equal part."
Theophylact"If it be asked here whether the Holy Ghost proceeds from the Son also… the Holy Spirit is not the Spirit of one, but of both; since Christ Himself said, 'The Spirit of your Father which speaks in you,' and the Apostle calls Him the Spirit of the Son."
St. AugustineThat the Son "sends" the Spirit shows the Son's equality with the Father, the Spirit is theirs to give. But notice the seed of the great division: Scripture says the Spirit "proceeds from the Father," and the question is whether He also proceeds from the Son. The Eastern Fathers (Theophylact here) keep to "from the Father," with the Son sending; the Western Fathers (Augustine here) conclude He proceeds "from both." Same verses, two emphases.
John 16:13-14 · KJVHowbeit when he, the Spirit of truth, is come, he will guide you into all truth… he shall glorify me: for he shall receive of mine, and shall shew it unto you.
The Spirit's work is not to start a new message but to complete and apply Christ's: He "guides into all truth," and crucially He "shall glorify Me, for He shall receive of Mine." The Spirit never points to Himself; He always magnifies Christ and unfolds what is His. That is the early church's test of anything claiming to be of the Spirit: does it glorify Jesus and accord with what He gave? The Spirit and the Son are never at cross-purposes.
Where the traditions diverge
On the heart of these chapters there is full agreement: the Holy Spirit is true God, a distinct Person ("another Comforter"), of one nature with the Father and the Son, the Spirit of truth who abides forever, teaches, and glorifies Christ. The Council of Constantinople (381) confessed Him "the Lord, the Giver of life, who proceedeth from the Father." The lasting divergence is over the filioque ("and the Son"): the Western church came to confess that the Spirit proceeds "from the Father and the Son" (Augustine's reading above), and added the phrase to the Creed; the Eastern church holds to "from the Father" alone (with the Son sending in time), and regards the unilateral addition to the Creed as the deeper grievance. This was a chief doctrinal cause of the great schism of 1054. The Fathers in the Catena hold both readings side by side, which is exactly why the difference is worth seeing honestly rather than pretending it away.
Patristic text from the Catena Aurea (public domain, transcription lightly corrected). Scripture in the King James Version; the plain-language lines are our own restatement, not the Fathers' words. See also the Trinity in Matt 28; this passage in the Study Bible; John 14 at BibleHub.