The selection of an apostle was conditioned not only upon divine appointment but upon having been an eyewitness to the resurrected Christ (Acts 1:21). Jesus speaks to the Apostles in Acts 1:8, saying,
… but you will receive power when the Holy Spirit has come upon you; and you shall be My witnesses both in Jerusalem, and in all Judea and Samaria, and even to the remotest part of the earth.” (cf. Luke 24:48,Acts 4:33, 13:31; 2 Peter 1:16; 1 John 1:1).
After the ascension, Matthias was chosen to replace Judas, who had hung himself. It was at this time that Luke records the prerequisites for being an apostle. Peter says,
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 (cf. John 15:27).
Ananias said to Paul, “The God of our ancestors has chosen you to know his will and to see the Righteous One and to hear words from his mouth. You will be his witness to all people of what you have seen and heard” (Acts 22:14-15; cf. 23:11, 26:16). Since Paul was not chosen with the twelve and did not follow Christ during his earthly stay, many would question his claim of being an apostle of Jesus Christ. Consequently, Paul would have to defend his apostleship (2 Cor. 10:1-12:21; Gal. 1-2; cf. 1 Cor. 9:1-15). He describes his selection as being “out of due season.” Nevertheless, he was chosen by the Lord who appeared to him on the road to Damascus and gave him his orders. His appearance to Paul gave proof for a risen Savior and qualified Paul as having seen the Christ. The content of his teaching was focused on this resurrection theme.