Then after the event, he appositely brings in the Prophet, saying, “For it is written in the Book of Psalms, Let his habitation be desolate, and let no man dwell therein”: this is said of the field and the dwelling: And his bishopric let another take; that is, his office, his priesthood. So that this, he says, is not my counsel, but His who has foretold these things. For, that he may not seem to be undertaking a great thing, and just such as Christ had done, he adduces the Prophet as a witness. “Wherefore it behooves of these men which have companied with us all the time.” Why does he make it their business too? That the matter might not become an object of strife, and they might not fall into contention about it. For if the Apostles themselves once did this, much more might those. This he ever avoids. Wherefore at the beginning he said, “Men and brethren. It behooves” to choose from among you. He defers the decision to the whole body, thereby both making the elected objects of reverence and himself keeping clear of all invidiousness with regard to the rest. For such occasions always give rise to great evils. Now that some one must needs be appointed, he adduces the prophet as witness: but from among what persons: “Of these,” he says, “which have companied with us all the time.” To have said, the worthy must present themselves, would have been to insult the others; but now he refers the matter to length of time; for he says not simply, “These who have companied with us,” but, “all the time that the Lord Jesus went in and out among us, beginning from the baptism of John unto that same day that He was taken up from us, must one be ordained to be a witness with us of His resurrection”: that their college (ὁ χορὸς) might not be left mutilated. Then why did it not rest with Peter to make the election himself: what was the motive? This; that he might not seem to bestow it of favor. And besides, he was not yet endowed with the spirit. “And they appointed two, Joseph called Barsabus, who was surnamed Justus, and Matthias.” Not he appointed them: but it was he that introduced the proposition to that effect, at the same time pointing out that even this was not his own, but from old time by prophecy; so that he acted as expositor, not as preceptor. “Joseph called Barsabus, who was surnamed Justus.” Perhaps both names are given, because there were others of the same name, for among the Apostles also there were several names alike; as James, and James (the son) of Alphæus; Simon Peter, and Simon Zelotes; Judas (the brother) of James, and Judas Iscariot. The appellation, however, may have arisen from a change of life, and very likely also of the moral character. “They appointed two,” it is said, “Joseph called Barsabus, who was surnamed Justus, and Matthias. And they prayed, and said; Thou, Lord, which know the hearts of all men, show whether of these two you have chosen, that he may take part of this ministry and Apostleship, from which Judas by transgression fell, that he might go to his own place.” (v. 24-25.) They do well to mention the sin of Judas, thereby showing that it is a witness they ask to have; not increasing the number, but not suffering it to be diminished. “And they gave forth their lots” (for the spirit was not yet sent), “and the lot fell upon Matthias: and he was numbered with the eleven Apostles.”
“Then,” it says, returned they unto Jerusalem from the mount called Olivet (Recapitulation), [“which is near to Jerusalem, at the distance of a sabbath-day's journey:”] so that there was no long way to go, to be a cause of alarm to them while yet trembling and fearful. “And when they had come in, they went up into an upper room.” They dared not appear in the town. They also did well to go up into an upper room, as it became less easy to arrest them at once. “And they continued,” it is said, “with one accord in prayer.” Do you see how watchful they were? “Continuing in prayer,” and “with one accord,” as it were with one soul, continuing therein: two things reported in their praise. [“Where they were abiding,” etc., to, “And Mary the Mother of Jesus and His brethren.”] Now Joseph perhaps was dead: for it is not to be supposed that when the brethren had become believers, Joseph believed not; he who in fact had believed before any. Certain it is that we nowhere find him looking upon Christ as man merely. As where His mother said, [“Your father and I did seek you sorrowing.” And upon another occasion, it was said,] “Your mother and your brethren seek you.” So that Joseph knew this before all others. And to them [the brethren] Christ said, The world cannot hate you, but Me it hates.
Again, consider the moderation of James. He it was who received the Bishopric of Jerusalem, and here he says nothing. Mark also the great moderation of the other Apostles, how they concede the throne to him, and no longer dispute with each other. For that Church was as it were in heaven: having nothing to do with this world's affairs: and resplendent not with wails, no, nor with numbers, but with the zeal of them that formed the assembly. They were “about an hundred and twenty,” it says. The seventy perhaps whom Christ Himself had chosen, and other of the more earnest-minded disciples, as Joseph and Matthias. There were women, he says, many, who followed Him. [“The number of the names together.] Together” they were on all occasions.
[“Men and brethren,” etc.] Here is forethought for providing a teacher; here was the first who ordained a teacher. He did not say, 'We are sufficient.' So far was he beyond all vain-glory, and he looked to one thing alone. And yet he had the same power to ordain as they all collectively. But well might these things be done in this fashion, through the noble spirit of the man, and because prelacy then was not an affair of dignity, but of provident care for the governed. This neither made the elected to become elated, for it was to dangers that they were called, nor those not elected to make a grievance of it, as if they were disgraced. But things are not done in this fashion now; nay, quite the contrary.— For observe, they were an hundred and twenty, and he asks for one out of the whole body: with good right, as having been put in charge of them: for to him had Christ said, “And when you are converted, strengthen your brethren.”
Source: Homilies on Acts (New Advent)