Paul of Tarsus
The wild card of this list, but give him fair consideration. Paul accomplished more with the few letters we have of his, to various churches in Asia Minor, Israel and Rome, than any other mortal person in the Bible, except Jesus himself. Jesus founded Christianity. But without Paul, the religion would have died in a few hundred years at best, or remained too insular to invite the entire world into its faith, as Jesus wanted.
About AD 49, after 14 years of preaching, Paul travelled to Jerusalem with Barnabas and Titus to meet with the leaders of the Jerusalem church -- namely Peter, James the Just, and the Apostle John -- an event commonly known as the Apostolic Council. Here the accounts of Acts (chapter 15) and Paul vary considerably: Acts states that Paul was the head of a delegation from the Antiochene church that came to discuss whether Christians should continue to observe Mosaic Law, most important of which were the practice of circumcision and dietary laws; Paul later said he had attended on his own initiative, concerned that the churches he had helped to found amongst the gentiles over the previous years might be excommunicated from the leading one at Jerusalem, and concerned to defend his belief that Christ's resurrection had freed Christian believers from the need to obey Mosaic Law.
Paul's conversion dramatically changed the course of his life. Through his missionary activity and writings he eventually transformed religious belief and philosophy around the Mediterranean Basin. His leadership, influence and legacy led to the formation of communities dominated by Gentile groups that worshiped the God of Israel, adhered to the "Judaic moral code", but relaxed or abandoned the ritual and dietary teachings of the Law of Moses, that these laws and rituals had either been fulfilled in the life of Christ or were symbolic precursors of Christ, all on the basis of Paul's teachings of the life and works of Jesus Christ and his teaching of a New Covenant (or "new testament") established through Jesus' death and resurrection. The Bible does not record Paul's death.
Paul is recognized by many Christians as a saint. Paul did much to advance Christianity among the gentiles, and is considered one of the primary sources of early Church doctrine. Some argue that it was he who first truly made Christianity a new religion, rather than a sect of Judaism.
He is especially impressive to have systematized these principles flawlessly, having never met Jesus in person, and in direct opposition to Peter and several other Disciples. Many theologists and experts on Christianity and its history even call Paul, and not Jesus, the founder of Christianity. That may be going a bit too far, but keep in mind that the Disciples intended to keep Christianity for themselves, as the proper form of Judaism, to which only Jews could convert.
Anyone could symbolically become a Jew by circumcision and obedience of the Mosaic Laws (every one of them, not just the Big Ten). Paul argued against this, stating that as Christ was the absolute greatest good that the world would ever see, and Almighty because he and the Father are one, then the grace of Christ is sufficiently powerful to save anyone from his or her sin, whether Jewish, Gentile or anything else. If the religion were to have lasted to present day without Paul’s letters championing the grace of Christ over the Law of Moses, Christianity would just a minor sect of Judaism.