Acts 2:41

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.

"Then they that gladly received his word were baptized; and the same day there were added unto them about three thousand souls." Think you not this cheered the Apostles more than the miracle?

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. This is the cause that our antagonist has so little trouble, for we get entangled in our own movements, and are continually thrown down.

Once there were three thousand - there were five thousand - and yet they had all one soul: but now each knows not his brother, and is not ashamed to lay the blame on the number, because it is so great! Yet he that has many friends is invincible against all men: stronger he than any tyrant.

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.

Those, therefore, who received his word were baptized. Another translation, according to the accuracy of the Greek truth, has it thus: Those indeed willingly accepting his word were baptized. By this interpretation it seems more clearly expressed that not some of those who had heard Peter's word, but all who had assembled to hear, willingly hearing this, were baptized.

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).