(Verse 21) - Wherefore of these men - Of those who had witnessed the life and works of Christ, and who were therefore qualified to discharge the duties of the office from which Judas fell. Probably Peter refers to the seventy disciples, (Luke 10:1-2).

Went in and out - A phrase signifying that he was their constant companion. It expresses in general all the actions of the life, (Ps. 121:8; Deut. 28:19; 31:2).

(Verse 22) - Beginning from the baptism of John - The words “beginning from” in the original refer to the Lord Jesus. The meaning may be thus expressed, “during all the time in which the Lord Jesus, beginning (his ministry) at the time when he was baptized by John, went in and out among us, until the time when he was taken up,” etc. From those who had during that time been the constant companions of the Lord Jesus must one be taken, who would thus be a witness of his whole ministry.

Must one be ordained - It is fit or proper that one should be ordained. The reason of this was, that Jesus had originally chosen the number twelve for this work, and as one of them had fallen, it was proper that the vacancy should be filled by some person equally qualified for the office. The reason why it was proper that he should be taken from the seventy disciples was, that they had been particularly distinguished by Jesus himself, and had been witnesses of most of his public life, (Luke 10:1-16). The word “ordained” with us has a fixed and definite signification. It means to set apart to a sacred office with proper forms and solemnities, commonly by the imposition of hands. But this is not, of necessity, the meaning of this passage. The Greek word usually denoting “ordination” is not used here. The expression is literally, “must one be, or become, a witness with us of his resurrection.” The expression does not imply that he must be set apart in any particular manner, but simply that one should be designated or appointed for this specific purpose, to be a witness of the resurrection of Christ.

(Verse 23) - And they appointed two - They proposed, or, as we should say, nominated two. Literally, they placed two, or made them to stand forth, as persons do who are candidates for office. These two were probably more distinguished by prudence, wisdom, piety, and age than the others, and they were so nearly equal in qualifications that they could not determine which was the best suited for the office.

Joseph called Barsabas  - It is not certainly known what the name Barsabas denotes. The Syriac word “Bar” means “son,” and the word “Sabas” has been translated “an oath, rest, quiet, or captivity.” Why the name was given to Joseph is not known but probably it was the family name - Joseph son of Sabas. Some have conjectured that this was the same man who, in Acts 4:36, is called Barnabas. But of this there is no proof. Some suppose that he was the son of Alpheus and brother of James the Less, and that he was chosen on account of his relationship to the family of the Lord Jesus.

Was surnamed Justus - Who was called Justus. This is a Latin name, meaning just, and was probably given him on account of his distinguished integrity. It was not uncommon among the Jews for a man to have several names, Matt. 10:3.

And Matthias - Nothing is known of the family of this man, or of his character, further than that he was numbered with the apostles, and shared their lot in the toils, the persecutions, and the honors of preaching the gospel to mankind.