So, Matthias filled the vacancy left by Judas. There was never again a need during the lifetime of those who would have qualified. When there was need of a replacement, there were none who could qualify.
The Apostle Paul played a unique and essential part in cementing the early Christian church. Note that I put Apostle in quotations this one time because it is important to note that Paul was never and could not ever be given such a position.
Before his dramatic conversion and taking a new name, Saul devoted his life during the ministry of Jesus to doing all that he could to persecute those who followed Jesus. He had been present and had consented to the stoning and death of Stephen, the first recorded martyr of the church [Acts 7:54-60]. Immediately the persecution spread as followers fled Jerusalem into the surrounding areas of Judea and Samaria [Acts 8]. With letters from the high priest, Saul headed for Damascus to bring back in chains those who had fled Jerusalem. As he approached Damascus his most dramatic conversion experience occurred [Acts 9:1-19].
Because of his steadfast and devoted service in his ministry to the gentiles, he was highly respected and loved. Even the Apostles referred to him as an apostle and Paul called himself an apostle. In a certain way, Paul became an interpreter of the New Law for the people of his day. He continued demonstrating and expanding the good news. His letters to those who were struggling to follow their newfound faith became a shining light for those who were seeking. His epistles recorded in the New Testament are the only items included that did not come from the original Apostles of the church.
Pauls letters, taken as a package, certainly does have what can be perceived as contradictions. He did speak on some matters that came from his humanity but we must remember that every human must do the same. He spoke of many subjects that were not addressed by Jesus in the four gospels -- and we must remember that Jesus was the WORD. Like all of us, Paul had his own monkeys on his back [which he mentioned but he never said what they were.]
Paul spoke to the people of his day on how they could conduct themselves in a manner to gain eternal life, just as we in our time seek the same goal promised by Jesus. In the New Law, some things practiced in the Old Law, such as circumcision, became an early question that Paul addressed [1 Corinthians 7:18-19]. Some say that Paul went too far; he addressed issues that had no bearing on the ministry of Jesus but we need consider the time of his ministry. Perhaps it was important then but in our times there is no real problem when a woman prays without a head covering or a man has long hair [1 Corinthians 1:5,14] or for a woman to speak in church [1 Corinthians 14:34].