What qualifications were required to replace Judas as one of the Twelve?
To replace Judas, the man had to have accompanied the apostles throughout Jesus' ministry, beginning from John's baptism until the day Jesus was taken up. He needed to have been a follower for Jesus' whole three years, witnessed the resurrection, and heard Jesus speak during the 40 days before His ascension.
Les explains that Peter, speaking to the 120 gathered in the upper room, set the qualification for Judas' replacement: he must be ordained as a witness with them of Jesus' resurrection. The man had to have accompanied them the whole time the Lord Jesus went in and out among them, from John's baptism until the day Jesus was taken up. That meant he had been converted from John's baptism, had followed Jesus throughout His whole three years of ministry, had witnessed His resurrection, and had heard Him speak during the 40 days before His ascension. Otherwise he was not qualified. Saul of Tarsus did not come close to any of this, because he had not become a believer until just then, had been an enemy, and was totally separated from the Twelve.