John Calvin Commentary Acts 1:15

"And in these days Peter stood up in the midst of the brethren, and said (and there was a multitude of persons [gathered] together, about a hundred and twenty)," — Acts 1:15 (ASV)

It was fitting that Matthias should be chosen for Judas's place, so that, through the treachery of one man, all that Christ had once appointed might not seem to have been nullified. He did not choose the twelve in the beginning without careful thought, as chief preachers of His gospel. For when He said that they should be judges of twelve tribes of Israel (Luke 6:13; John 6:70), He shows here that it was done with deliberate purpose, so that they might gather the tribes of Israel to one faith. But after the Jews had refused the grace offered to them, it was necessary that the Israel of God should be gathered from all countries.

This, therefore, was, as it were, a holy number, which, if it had been diminished by Judas's wickedness, then the preaching of the gospel would have had less credibility to this day, if its beginning had been imperfect. Therefore, although Judas, as much as he could, would have frustrated Christ's purpose, it nevertheless stood firm and stable. He perished as he deserved, yet the order of the apostles remained complete and intact.

The company of names. It is uncertain whether he means the men who alone properly have the name (since women are included under the name of men); or whether he takes 'names' simply for all the individuals, as the Hebrews call them 'souls.' It may also be questioned whether they were accustomed to visit daily the room in which the apostles lived, or if they continually lived there with them.

For the place was hardly able to contain such a large multitude and meet all their necessary needs. Certainly, it seems more likely to me that Luke expresses their number here so that we may know that they were all gathered together when Peter delivered this sermon.

From this we may infer that they were not always present there. Although I do not dare to state anything certain about this matter, yet, persuaded by a probable conjecture, I rather incline to the view that the church was gathered together then because they had to discuss a serious matter, and the word rising, [standing up.] also points to this.