It was meet that Matthias should be chosen into the place of Judas, lest, through the treachery of one man, all that might seem to have been made of none effect which Christ had once appointed. He did not unadvisedly choose the twelve in the beginning, as principal 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 showeth here that it was done of set purpose, that they might gather together the tribes of Israel unto one faith. But after that the Jews had refused the grace offered unto them, it was behoveful that the Israel of God should be gathered together out of all countries. This, therefore, was, as it were, a holy number, which, if it should have been diminished through the wickedness of Judas, then should the preaching of the gospel both have had, and also have, less credit at this day, if the beginning thereof had been imperfect. [1] Although, therefore, Judas would (as much as in him lay) have disappointed the purpose of Christ, yet nevertheless it stood firm and stable. He perished as he was worthy, yet did the order of the apostles remain whole and sound. The company of names It is uncertain whether he meaneth the men who only have the name properly, seeing the women are comprehended under the name of the men; or whether he taketh names simply for all the heads, as the Hebrews call them souls. This may also be called in question, whether they were wont daily to frequent that parlor in which the apostles did dwell, or they did continually dwell there with them. For the place was scarce able to contain so great a multitude, to serve them for all necessary uses. Surely it seemeth to me a thing more like to be true, that Luke doth in this place express the number of them, that we may know that they were all gathered together when Peter made this sermon. Whereby we may guess that they were not always present there. Although I dare not affirm any certain thing concerning this matter, yet being moved with a probable conjecture, I do rather lean unto this part, that the church was gathered together them because they had to intreat of a serious matter, and to this end also tendeth this word rising, [standing up.]

Footnotes

1 - "Ut ita loquar claudicaret," had been, so to speak, lame, defective.

In those days - On one of the days intervening between the ascension of Jesus and the day of Pentecost.
Peter stood up - Peter standing up, or rising. This is a customary expression in the Scriptures when one begins to do a thing, Luke 15:18. The reason why Peter did this may be seen in the notes on Matthew 16:16-17. It is not improbable, besides, that Peter was the most aged of the apostles; and from his uniform conduct we know that he was the most ardent. It was perfectly characteristic, therefore, for him to introduce the business of the election of a new apostle.
The disciples - This was the name, which was given to them as being learners in the school of Christ. See the notes on Matthew 5:1.
The number of the names - The number of the persons, or individuals. The word "name" is often used to denote "the person," Revelation 3:4; Acts 4:12; Acts 18:15; Ephesians 1:21. In Syriac it is, "The assembly of people was about an hundred and twenty." This was the first assembly convened to transact the business of the church; and it is not a little remarkable that the vote in so important a matter as electing apostle was by the entire church. It settles the question that the election of a minister and pastor should be by the church, and that a pastor should not be placed over a church by a patron, or by an ecclesiastical body. If a case could ever occur where it would be right and proper that one should be selected to exercise the office of a minister of Christ by the ministry only, the election of one to fall the office of an apostle was such a case. And yet in this the entire church had a voice. Whether this was all the true church at this time does not appear from the history. This expression cannot mean that there were no more Christians, but that these were all that had convened in the upper room. It is certain that our Saviour had, by his own ministry, brought many others to be his true followers. Compare 1-Corinthians 15:6.

In the midst of the disciples - Μαθητων; but instead of this, αδελφων, brethren, is the reading of ABC, a few others, with the Coptic, Ethiopic, Armenian, and Vulgate. This seems the best reading, because of what immediately follows; for it was not among the disciples merely that he stood, but among the whole company, which amounted to one hundred and twenty. It is remarkable that this was the number which the Jews required to form a council in any city; and it is likely that in reference to this the disciples had gathered together, with themselves, the number of one hundred and twenty, chosen out of the many who had been already converted by the ministry of our Lord, the twelve disciples, and the seventy-two whom he had sent forth to preach, Luke 10:1, etc., thus they formed a complete council in presence of which the important business of electing a person in the place of Judas was to be transacted.

(6) And in those days Peter stood up in the midst of the disciples, and said, (the number of (p) names together were about an hundred and twenty,)
(6) Peter is made the spokesman and interpreter of the whole company of the Apostles, either by secret revelation of the Holy Spirit, or by the express judgment of the congregation.
(p) Because men are commonly referred to and enrolled by their names.

And in those days Peter stood up,.... That is, in one of the days after Christ's ascension, and before the day of Pentecost, whilst the disciples were waiting for the promise of the Spirit. The Ethiopic version reads, "on that day"; as if it was the same day they came first into Jerusalem, and went into the upper room; and which is likely enough; for no time was to be lost in choosing one in the room of Judas; when Peter, not only as a forward person, and who had been used to be the first mover and actor in any affair; but as willing to show his zeal for Christ, whom he had so lately denied, and as being the senior man in company, as well as the minister of the circumcision, rises, and stands up, as persons used to do, when about to make an oration, and in respect and reverence to the persons addressed:
in the midst of the disciples; not only the other ten, but the whole hundred and twenty. The Alexandrian copy, and some others, read, "in the midst of the brethren", and so reads the Vulgate Latin version; and the Ethiopic version, "in the midst of his own brethren",
and he said what is expressed in the following verses, which before the historian relates, he inserts in a parenthesis this clause,
the number of the names; that is, of persons; see Revelation 11:13. Some copies read, "of the men", and so the Vulgate Latin and Syriac versions; who
together, all put together in one sum, or as meeting together in one and the same place, or as agreeing in the same faith and judgment, so the Arabic version, "and there was there a company whose names and wills agreed in this same opinion"; they were all in one place, and of the same mind; and the sum of them
were about an hundred and twenty; among whom were the eleven apostles, and seventy disciples, which made eighty one; so that there were thirty nine persons more in this company: not that it is to be thought that these were all that were in Jerusalem that believed in Christ; but these were the number of the persons that met and embodied together in a church state, and who not only gave themselves to the Lord, but to one another, by the will of God; and their names being taken and registered, the historian calls the account of them, the number of the names, and not persons; though he means persons. This was a number pretty famous among the Jews; the sanhedrim of Ezra, called the men of the great synagogue, consisted of an "hundred and twenty elders"; the last of which was Simeon the just, and he comprehended the hundred and twenty (h). And such a number was requisite for a sanhedrim in any place; it is asked,
"how many must there be in a city, that it may be fit for a sanhedrim? "an hundred and twenty"; R. Nehemiah says two hundred and thirty (i):
but the decision is according to the former: hence they say (k), that "they fix in every city in Israel, where there is an "hundred and twenty", or more, a lesser sanhedrim.---A city in which there is not an hundred and twenty, they place three judges, for there is no sanhedrim less than three.
(h) Juchasin, fol. 13. 2. Bartenora in Pirke Abot, c. 1. sect. 1. Elias Levit. prefat. 3. ad Sepher Masoret. (i) Misn. Sanhedrin, c. 1. sect. 6. Vid. Maimon. & Bartenora in. ib. & T. Bab. Sanhedrin, fol. 17. 2. (k) Maimon. Hilchot Sanhedrin, c. 1. sect. 3, 4.

The great thing the apostles were to attest to the world, was, Christ's resurrection; for that was the great proof of his being the Messiah, and the foundation of our hope in him. The apostles were ordained, not to wordly dignity and dominion, but to preach Christ, and the power of his resurrection. An appeal was made to God; "Thou, Lord, who knowest the hearts of all men," which we do not; and better than they know their own. It is fit that God should choose his own servants; and so far as he, by the disposals of his providence, or the gifts of his Spirit, shows whom he was chosen, or what he has chosen for us, we ought to fall in with his will. Let us own his hand in the determining everything which befalls us, especially in those by which any trust may be committed to us.

in those days--of expectant prayer, and probably towards the close of them, when the nature of their future work began more clearly to dawn upon them, and the Holy Ghost, already "breathed" on the Eleven (John 20:22), was stirring in Peter, who was to be the leading spirit of the infant community (Matthew 16:19).
the number . . . about an hundred and twenty--Many, therefore, of the "five hundred brethren" who saw their risen Lord "at once" (1-Corinthians 15:6), must have remained in Galilee.

And in those days. The interval of ten days.
Were about a hundred and twenty. This was the number of disciples at Jerusalem, but not all who were then disciples. See 1-Corinthians 15:6.

The number of persons together - Who were together in the upper room. were a hundred and twenty - But he had undoubtedly many more in other places; of whom more than five hundred saw him at once after his resurrection, 1-Corinthians 15:6.