Acts 2:6

Since the event had taken place in a house, of course they came together from without. The multitude was confounded: was all in commotion. They marvelled; "Because that every man heard them speak in his own language. And they were amazed," it says, "and marvelled, saying one to another, Behold, are not all these which speak Galileans?" They immediately turned their eyes towards the Apostles.

"Behold, are not all these which speak Galileans?" For indeed this was confessed. "And how hear we" so much did the sound alarm them. "Every man in our own tongue," etc. for it found the greater part of the world assembled there.

The multitude was perplexed or disturbed, for reasonably they supposed that wondrous things were being shown to them, on account of the bold deeds done concerning Christ, and conscience troubled their souls. And some were murderous in this way; but the devout ones were amazed, recognizing them as Galileans yet hearing them speak in their native tongues.

But the apostles knew what they were saying, namely the mighty works of God; as for the languages, they did not know them, except that the men who heard were devout, for example some Scythian by chance, or an Indian who understood the Scythian, or that the apostles spoke in the dialect of the Indians.

Because each one was hearing them speak in his own language. It is asked in this place how each one heard them speaking the great things of God in his own language: whether those who were speaking in different languages were delivering what was being said in the language of each one, that is, whether each of them now spoke in this, now in another language, thus running through all, or it was more marvelous in this, that the speech of those who were speaking, whatever language had been pronounced, was understood by each one hearing according to his own language, that (for example) when any one apostle was teaching in the church (for it was necessary that one should speak while the others were silent, and one speech should reach the hearing of all), that same speech would have such a force in itself, that although there were listeners of different nations, each one would receive the hearing and grasp the understanding of that one speech pronounced by the apostle according to his own language. Unless perhaps it seemed more wonderful that this was a miracle of the listeners rather than of the speakers.

Since each one heard them speaking in his own language, they were all amazed and astonished, saying, "Are not all these who are speaking Galileans?" I know I have been criticized by some for saying that this sentence can be understood in two ways, or rather for asking how it should be understood. To them, I briefly respond that everything I wrote about the same sentence in my previous volume was not expressed from my own thought but taken from the words of the holy and utterly irreproachable teacher, that is, Gregory of Nazianzus. It is indeed clear that the apostles, filled with the Holy Spirit, spoke in all tongues, nor is it allowed for any of the faithful to doubt this. But how they spoke is rightly questioned, namely whether the speech of the apostles had such power that everyone who knew different languages could equally understand it when heard, or if whoever spoke, as it was necessary for someone to speak among such a multitude, while the rest were silent, at first made his speech in Hebrew for the Hebrews, with the rest not knowing what he said. Then for the Greeks, he spoke in Greek while the others waited. Then for the Parthians, and after them the Medes, and thus for the Elamites, and those peoples enumerated in order, he spoke in their own language, with each group waiting and remaining silent until their turn came, so they could understand what was said and thus give assent to the words of the teaching in faith. However, Luke reports Peter speaking to the crowds, but does not report him repeating the same things a second or third time. Instead, he notes that they, heeding the counsel of salvation, were consecrated in the mysteries of the Christian faith. I do not think it would be wrong for someone to believe that both things could have happened: that the apostles, filled with the Holy Spirit, could have understood and spoken the languages of all nations, and also that their words, by a greater miracle, could have been equally understood by all who heard them, regardless of the language in which they were spoken.

"When this sound occurred, the multitude gathered and was confounded," that is, was confused, was amazed. Since this happened in a house, it is evident that the people came running from outside.
"For every man heard them speak in his own dialect." They knew that the believers, and especially the apostles (since those gathered directed their gaze more upon the apostles), were Galileans; and yet they spoke in an astonishingly great number of languages.
"The people… were confounded," naturally, because those who had gathered supposed that the event threatened them for the evil deed they had committed against Christ; and their conscience was tearing their souls apart, since the killing of Christ was still fresh in their memory and they were afraid of everything. But this strengthened the apostles, since the listeners themselves made it known to them that this was a miraculous gift, because the apostles did not realize that a certain expression was Parthian, but learned this from the listeners. He also mentions hostile nations — Cretans, Arabians, and others; this was a symbol that they would conquer all, since in Jerusalem there were many captives. Thus, the testimony came from everywhere: from citizens, from foreigners, and from proselytes.