‘Of the men therefore who have kept company with us all the time that the Lord Jesus went in and went out among us, beginning from the baptism of John, to the day that he was received up from us, of these must one become a witness with us of his resurrection.'
The credentials for the replacement, for a member of ‘the twelve', is made clear (which were in fact stricter than the ones Jesus had required for some of the original twelve). Such a one was to be someone who had been a disciple right from the beginning of Jesus' ministry when John was baptising and had travelled with Him extensively, ‘going in and out' among the disciples and being with them continually, and being a witness of the resurrection up to this very time of Jesus being received up. He was to have been an eyewitness and direct hearer of all that Jesus had done from the beginning, so that he could be a true witness.
‘Went in and went out among us.' For the phrase compare Acts 9:28; Deu 31:2; 2 Samuel 3:25; Psalms 121:8. It involves regular companionship and association.
This requirement confirms that the twelve could not continually to be maintained. Once those who had been with Jesus from the time of His baptism had died out it would have been impossible anyway. And the later acceptance of Paul as an Apostle, on different grounds, stresses the uniqueness of Apostleship. But he too recognised the necessity that he had seen the risen Lord, as one ‘born out of due time' (1 Corinthians 15:8). Being able to be a witness to the resurrection was thus seen as vitally essential.