To initiate the age of grace, God saves the chief of sinners as a pattern of longsuffering "to them that should hereafter believe on [Jesus Christ] to life everlasting (1 Tim. 1:15-16). That this is to be an age of longsuffering and grace is demonstrated in a marvelous way by the salvation of the chief of sinners and by his subsequent calling to proclaim the gospel of that grace (Acts 20:24).
So Paul, the chief of sinners, is chosen as point man for the grace gospel. But what about the kingdom gospel that preceded (Mt. 3-4, 9-10; Acts 1-12) and follows (Mt. 24:14) his? Who is the point man for that gospel? Well, you might say John Baptist or the Lord Jesus Christ himself, but I really think that the point man of the kingdom gospel is none other than Simon Peter, whom the Lord gave the keys to the kingdom of heaven to in Mt. 16: 19, and who emerges as the chief kingdom apostle before and after the Lord's passion.
There is however, an impasse in Peter's leadership...when he denies the Lord (Mark 14:72). Note carefully in Mk. 16:7 where Christ distinguishes Peter from the disciples. This is not an honour, but pointing out that he has lost his discipleship. Judas loses his permanently (Acts 1:20, 25), but Peter's is obviously temporary, since he clearly emerges as the leader of the twelve in Acts 1-12.
How was he restored? Please read John 21:15-19, where Christ asks him three times if he loves him, once for each of Peter's earlier denials (dabbling with "agapeo" and "phileo" will not show you this truth). Where I'm going with this is that the point man for the kingdom gospel is a man who actually fell from his discipleship by denying the Lord, something that the other ten (besides Judas, a devil--John 6:70), did not do. So again we see the grace of God, taking the kingdom apostle who falls the deepest and making him the chief of "the twelve apostles of the Lamb" (Rev. 21:14). No wonder this other "chief of sinners" exalts the Lord as "the God of all grace" (1 Peter 5:10), a title unique to Simon Peter, who experienced that grace very deeply.