Albert Barnes Commentary Acts 1:21-22

"Of the men therefore that have companied with us all the time that the Lord Jesus went in and went out among us, beginning from the baptism of John, unto the day that he was received up from us, of these must one become a witness with us of his resurrection." — Acts 1:21-22 (ASV)

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

Went in and out. This phrase signifies that He was their constant companion. It expresses, in general, all the actions of life (Psalms 121:8; Deuteronomy 28:19; Deuteronomy 31:2).

Beginning from the baptism of John. The words "beginning from," in the original Greek, refer to the Lord Jesus. The meaning can be expressed this way: "During all the time in which the Lord Jesus—beginning His ministry at the time He was baptized by John—went in and out among us, until the time He was taken up," and so on. From those who had been the constant companions of the Lord Jesus during that time, one must be chosen who would thus be a witness of His whole ministry.

Must one be ordained. It is fitting or proper that one should be ordained. The reason for 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 breach 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, commanded to preach, endowed with various powers, and had been witnesses of most of His public life (Luke 10:1–16).

The word ordained, for us, has a fixed and definite meaning. It denotes being set apart to a sacred office with the proper form and solemnities, commonly by the imposition of hands. However, this is not necessarily the meaning of this passage. The Greek word usually denoting ordination is not used here.

The expression is, literally, "must one be, or become—genesyai—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.