I’m sure you know that there are different opinions concerning the disciples’ selection of Matthias. The disciples were told to “wait” until the Spirit was sent, and yet they quickly went ahead and chose a replacement for Judas. They did so out of a sense of urgency, since they thought the kingdom may come at any time. Matthias is selected, and yet we really don’t hear of him again, but later on Paul is raised up, and we don’t cease hearing of him, or from him. This certainly raises the question as to whether or not Paul was God’s replacement for Judas, while Matthias was the disciples’ choice.
This issue is dealt with in greater detail in Bob Deffinbaugh’s series entitled, Acts: Christ at Work Through His Church.