John Chrysostom

They heard, and were forthwith baptized. They did not speak those cold words which we do now, nor did they contrive delays; and yet they had heard all the requirements: but that word, "Save yourselves from this generation," made them to be not sluggish; rather they welcomed the exhortation; and that they did welcome it, they proved by their deeds, they showed what manner of men they were. They entered at once the lists, and took off the coat; whereas we do enter, but we intend to fight with our coat on. …

Homily on Acts 7 ↗

Theophylact of Ohrid

Here is the fulfillment of the prophecy which says: "Who has heard such a thing? Who has seen such things? Shall a land be born in one day? Shall a nation be brought forth in one moment? For as soon as Zion was in labor, she brought forth her sons" (Is. 66:8).

Commentary on Acts ↗

Irenaeus

For this reason, too, did the apostles, collecting the sheep which had perished of the house of Israel, and discoursing to them from the Scriptures, prove that this crucified Jesus was the Christ, the Son of the living God; and they persuaded a great multitude, who, however, [already] possessed the fear of God. And there were, in one day, baptized three, and four, and five thousand men.

Against Heresies Book IV ↗

Bede

And about three thousand souls were added that day. Where the baptism of the Church was first celebrated, divine mercy, by careful plan, gathered a number of souls equal to the confession of the Holy Trinity. And Moses indeed, on the fiftieth day of Passover when the law was given, commanded the solemnity of the first fruits to be initiated; but now, with the Holy Spirit coming, not sheaves of wheat, but souls were consecrated as first fruits to the Lord.

Commentary on Acts ↗