John 12:29

How does God speak? Does He have a physical voice? Not at all, but He pours forth oracles with a certain superior power than a physical voice could have. The prophets heard this voice; the faithful hear this voice, but the wicked do not understand. Finally, in the Gospel you have, because the evangelist heard Him saying: "And I have glorified, and will glorify again" (John 12:28); but the Jews did not hear. For they said: "It thundered" (Ibid., 29). So there, just as you have above, because God, who was perceived as walking, did not walk, so God, who did not speak, was heard speaking.

(Tr. lii. 4) I have glorified it, i. e. before I made the world; and will glorify it again, i. e. when Thou shalt rise from the dead. Or, I have glorified it, when Thou wast born of a Virgin, didst work miracles, wast made manifest by the Holy Ghost descending in the shape of a dove; and will glorify it again, when Thou shalt rise from the dead, and, as God, be exalted above the heavens, and Thy glory above all the earth.
The people therefore that stood by and heard it, said that it thundered.

"Then came there a voice from heaven, [saying], I have both glorified it, and will glorify it again." "I have both glorified it," before I created the world, "and I will glorify it again," when He shall rise from the dead and ascend into heaven. It may also be otherwise understood. "I have both glorified it," when He was born of the Virgin, when He exercised miraculous powers; when the Magi, guided by a star in the heavens, bowed in adoration before Him; when He was recognized by saints filled with the Holy Spirit; when He was openly proclaimed by the descent of the Spirit in the form of a dove, and pointed out by the voice that sounded from heaven; when He was transfigured on the mount; when He wrought many miracles, cured and cleansed multitudes, fed so vast a number with a very few loaves, commanded the winds and the waves, and raised the dead; "and I will glorify it again;" when He shall rise from the dead; when death shall have no longer dominion over Him; and when He shall be exalted over the heavens as God, and His glory over all the earth.

"The people therefore that stood by, and heard it, said that it thundered: others said, An angel spake to Him. Jesus answered and said, This voice came not because of me, but for your sakes." He thereby showed that the voice made no intimation to Him of what He already knew, but to those who needed the information. And just as that voice was uttered by God, not on His account, but on that of others, so His soul was troubled, not on His own account, but voluntarily for the sake of others.

(Moral. xxviii.) When God speaks audibly, as He does here, but no visible appearance is seen, He speaks through the medium of a rational creature: i. e. by the voice of an Angel.

God speaks in words by an Angel, when nothing is displayed in outward appearance, but the words of the Heavenly saying are heard; as on the Lord saying, "Father, glorify Thy Son, that Thy Son may glorify Thee;" it is immediately replied, "I have glorified, and will glorify Him again." For God, Who speaks without time, by the power of inward impulse, uttered not in time that voice by His own Substance, which voice, circumscribed by time, He made plain by human words. But speaking doubtless from heavenly places, He fashioned, by the ministry of a rational creature, those His words which He wished to be heard by men.

Since very many were crude and ignorant, they took the voice for thunder, even though the voice was articulate and very clear. For they quickly forgot the words of the voice, retaining only its echo.
Others remembered the very words of the voice: "I have both glorified it and will glorify it again," yet not understanding the meaning of these words, they thought that an angel had spoken to Him, and therefore these words, as spoken by an angel, were incomprehensible to them.

Next we see the opinion of the crowd, which was wondering about the voice, The crowd standing by heard it and said. In this crowd, as in every other, some were dull and slow to understand, and others were more perceptive; yet all of them failed to identify the voice. Those who were slow and carnal only heard it as a sound; so they said that it had thundered. Still, they were not entirely mistaken, for the Lord's voice was thunder, both because it had an extraordinary meaning, and because it contained very great things: "How small a whisper do we hear of him! But the thunder of his power who can understand?" (Job 26:14); "The voice of your thunder" (Ps 76:19).

Those who were keener discerned that the sound was a voice, pronouncing words and having a meaning; so they said someone was speaking. But because they thought that Christ was merely human they erred, attributing these words to an angel. So they said that, An angel has spoken to him. They were under the same error as the devil, who thought that Christ needed the help of the angels: thus he said: "He will give his angels charge of you" (Mt 4:6). But he did not need to be guarded and helped by angels; rather, he is the one who glorifies and guards the angels.