12 I have yet many things to say to you: but you cannot bear them now. 13 But when he, the Spirit of truth, is come, he will teach you all truth. . . .
Didymus: His hearers had not yet attained to all those things which for His name’s sake they were able to bear; so, revealing lesser things, He puts off the greater for a future time, such things as they could not understand till the Cross itself of their crucified Head had been their instruction. As yet they were slaves to the types, and shadows, and images of the Law, and could not bear the truth of which the Law was the shadow. But when the Holy Ghost came, He would lead them by His teaching and discipline into all truth, transferring them from the dead letter to the quickening Spirit [Rom 7:6], in Whom alone all Scripture truth resides.
13 . . . For he shall not speak of himself; but what things soever he shall hear, he shall speak; . . .
Didymus: He shall not speak of Himself, i.e., not without Me, and Mine and the Father’s will: because He is not of Himself, but from the Father and Me. That He exists, and that He speaks, He has from the Father and Me. I speak the truth; i.e., I inspire as well as speak by Him, since He is the Spirit of Truth.
Augustine: This is like what He said of Himself above, i.e., I can of Mine own Self do nothing; as I hear I judge [John 5:30]. . . . The Son is born of the Father, and the Holy Ghost proceeds from the Father. . . . To hear is with Him to know, to know to be. As then He is not from Himself, but from Him from Whom He proceeds, from Whom His being is, from the same is His knowledge. From the same therefore His hearing. The Holy Ghost then always hears, because He always knows.
Nor let the use of the future tense perplex you; that hearing is eternal, because the knowledge is eternal. To that which is eternal, without beginning, and without end, a verb of any tense may be applied.
13 . . . and the things that are to come, he shall shew you.
Didymus: By the Spirit of truth too the knowledge of future events has been granted to holy men. Prophets filled with this Spirit foretold and saw things to come, as if they were present.
Bede: It is certain that many filled with the grace of the Holy Spirit have foreknown future events. But as many gifted saints have never had this power, the words, He will shew you things to come, may be taken to mean, bring back to your minds the Joys of your heavenly country.
Chrysostom: He raised their spirits; for there is nothing for which mankind so long, as the knowledge of the future. He relieves them from all anxiety on this account, by showing that dangers would not fall upon them unawares.
Scripture from the Douay-Rheims Bible. Commentary from St. Thomas Aquinas, Catena Aurea, Vol. IV (London: Rivington, 1845).