Paul declared himself to be an apostle (1 Corinthians 9: 2; 15: 9; Galatians 1: 17). An apostle is one who is sent by God. Jesus sent Paul to declare the gospel, so Paul is correct in his self-assessment. And because Paul is the most prominent writer in the New Testament, there are some who believe and claim that Paul was God’s choice to replace Judas Iscariot as the 12th apostle.
The problem with that is Paul did not meet the criteria set forth for the foundational, apostolic group. In Acts 1: 21, 22, the new apostle’s qualifications had to be that he had been associated with the group during the entire time of Jesus’ earthly ministry, and he had to have been a witness of Jesus’ resurrection and ascension.
Furthermore, God’s sovereign will is ignored by the supporters of Paul’s inclusion within the twelve. The eleven selected two men who met the above requirements— Matthias and Joseph called Barsabbas. The eleven prayed for God’s guidance. Afterwards, they cast lots. As a result, Matthias was chosen. If God had wanted Paul instead, He would have directed the eleven to wait and to come up with a method other than the biblically allowed method of casting lots in order to make a decision (Proverbs 16: 33).
There is no arguing that Paul is a prominent member of the body of Christ. In fact, other than Jesus, I think it’s safe to say that he may be the most prominent. Along with that, there is no argument that Matthias is never mentioned again. However, that doesn’t take away from Matthias’ legitimacy. After all, except for a few, the same can be said about most of the other apostles.
Unless I’ve missed something, based squarely on what’s been presented, it seems more than likely that it’ll be Matthias’ name, rather than Paul’s, written on one of the twelve foundations of the heavenly Jerusalem (Revelation 21: 14).