Acts 1:21

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.

If not even a successor to Judas could be named an apostle except from among those who were constant companions of Jesus from John's baptism until the resurrection of Christ, how is it possible that any person in subsequent ages should be hailed as an apostle?

These two verses shed light upon two of the most important subjects in the New Testament, (1) the qualifications of an apostle, and (2) the purpose of an apostle, that of witnessing the resurrection of Christ.

It should be noted that death did not remove Judas from his office; it was his betrayal of Jesus that removed him. When James was executed by Herod (Acts 12:2), no successor was chosen. Moreover, Christ had promised the Twelve that "in the times of the regeneration" (that is, this present dispensation) they would reign concurrently with Christ, "sitting upon twelve thrones and judging the twelve tribes of (spiritual) Israel" (Matthew 19:28). Therefore, all of the Twelve except Judas are still in office, all thought of a successor to any of them being absolutely an error.

Went in and went out ... This is an idiom. "It is a familiar Hebrew phrase for the whole of a man's life and conduct."[24] Luke used it again in Acts 9:28.

Witness with us of his resurrection ... The prime function of an apostle was that of a witness of Christ's resurrection; and, in the history of the world, there was never any such thing as a person not a witness becoming a successor to a witness. In the very nature of witnesses, there can be no such thing as a successor.

Also, here is identified the principal doctrine of Christianity, namely, the resurrection of our Lord. As Hervey noted:

The resurrection of Christ from the dead thus appears to be a cardinal doctrine of the gospel. The whole truth of Christ's mission, the acceptance of his sacrifice, the consequent forgiveness of sins, and all man's hopes of eternal life, turn upon it.[25]

[24] E. H. Plumptre, The Acts of the Apostles (Grand Rapids, Michigan: Zondervan Publishing House, 1959), p. 5.

[25] A. C. Hervey, The Pulpit Commentary (Grand Rapids, Michigan: Wm. B. Eerdmans, Publishers, 1950), Acts, p. 6.