For many Jews and Christians, the answer has been Paul. Frederick Nietzsche called Paul "The First Christian;" George Bernard Shaw evaluated his contribution as "a monstrous imposition," and Adolf von Harnack, the great 19th century church historian, called him, simply, "The Founder of Christian Civilization."
That opinion extends to popular imagination. A line heard frequently in my youth went, "Jesus was a nice Jewish boy; Paul was an apostate who organized Christianity into a religion in opposition to Judaism." Since becoming an historian of the period, I have learned different.
It's probably more correct to say that the second founder of Christianity was not Paul, but rather the apostles who saw the earthly Jesus. Paul, writing in the mid-decades of the first century C.E. before the Gospels were set down, rarely gives us any of Jesus' words or any information about the man Jesus. Instead, he gives us a record of his own spiritual life and his faith in the crucified Messiah. He gives primacy to his personal relation to Christ through revelations and visions.
The Gospel writers, influenced by those who knew Jesus, differed with Paul's ideas about resurrected bodies and Jesus' Second Coming. We might even say that the Gospels were redacted in part to "flesh out" Paul's writings. Within the Gospels we see the themes of immortality and resurrection that Paul provides, but also a struggle to keep any extraneous notions of immortality out of the story of the Christ. In the end, the Gospel writers won. Paul's more mystical view of Christianity did not predominate.
To understand how the Gospels and Paul differ, we need to look at Paul's understanding of resurrection bodies--both Jesus' and that of believers. Paul's mystical faith depended on identifying his salvation with the risen Messiah. The identification was not metaphorical, but a real transformation in the same form as the risen Christ in heaven (the fancy word for this is 'symmorphosis'). For Paul, Jesus' resurrected body was a spiritual body (in the original Greek, soma pneumatikon
Just as Christ's body is spiritual, says Paul, so our flesh and blood bodies will not enter the resurrection. Flesh and blood cannot inherit the kingdom of God, but the risen Christ is a "body of glory" (1 Cor 15:30). And so the new body which God gives His faithful in the resurrection will be a pneumatic or spiritual body augmented by the Spirit of God. This does not do away with the body but transforms
it to a spiritual substance, no longer flesh and blood.
It's not that Paul didn't believe in Jesus' physical resurrection. It's rather that he never saw the need to be more specific about it. What he cared about was the glorified spiritual body because that was his experience, gained in revelations and visions. It is his religious experience, not philosophy, which he describes in his letters.
The destiny of all believers will be this transformation into a spiritual substance. Paul makes an explicit analogy with the stars (1 Cor. 15:41), which are both spiritual and bodies at the same time. This links the transformation process with the Old Testament figure Daniel (Daniel 12 describes the wise as transformed into stars). People who are transformed in Christ will have the same substance as the stars, which are luminous and spiritual in nature.
This is, for Paul, the very fulfillment of the end of time. It is a spiritual transformation in which the flesh is left behind. As the Eschaton approached and as humans learned to be in Christ, the kingdom of God would be actualized. And from his visions, Paul knows that the process of transformation into a glorified, spiritual body has already begun and will be completed at the last trumpet.
Such a transformation is not clearly present in the Gospels, written in the decades after Paul's letters. The Gospels reflect the religious needs of a later generation of Christians and their reflections on the issue of faith, religious authority, and the afterlife. The Gospels are devices for the mission of the church, a different and broader mission than envisioned by Paul.
The Gospels may actually attempt to correct some of Paul's notions. It took centuries before Christianity could find an acceptable formula for incorporating immortality of the soul into the story of Christ's sacrificial death and resurrection. During the first centuries of Christianity, the proto-orthodox church was thinking of ways to fight against such notions of immortality, which were characteristic of groups like the "Gnostics." Immortality of the soul was considered dangerous because it, like being a ghost or a "spirit," was available to everyone naturally, even without Jesus' ultimate sacrifice on the cross. If Jesus' post-resurrection body were merely ghostly or spiritual or "his immortal soul," no one needed to be a believer in Christianity. That post-mortal form was available to everyone already in popular thinking.
Paul became a Christian because of his vision of the risen Christ in a glorified body. The earthly Jesus, conversely, had little to do with Paul's faith, for he never met the man Jesus. Since Paul's faith was based upon his own experience and visionary revelations, faith, vision, and knowledge were all deeply interwoven for him. His inward life was parallel to and indicative of the redemption of the world.
Not so for those working in the apostolic tradition into which Paul fought so hard to be included. Their writing starts appearing in the 70s of the first century and continues through the beginning of the second century. They knew something that Paul could not know--namely that Jesus' Second Coming would not happen in the first generation after Jesus--and they adapted to that fact. Part of this meant correcting for Paul's overly spiritual religious life and defending Christianity against the errors of even more radical thinkers.
For these proto-orthodox Christians, the inward process became secondary and the redemption of the world primary. Faith meant belief in those who had received their knowledge from the original apostles, who sat at the feet of Jesus and witnessed his life, death, and resurrection.
Part of faith was the trust that apostolic succession was the correct, "orthodox" teaching. Faith for them meant the teaching, ministry, and education that started with the earthly Jesus and continued through the apostles, but not the visionary knowledge of Paul or anyone else who claimed to have met Jesus in visions.
For the evangelists of that apostolic lineage, Jesus' resurrected body was a literal, physical body revivified, not a spiritual body. Their approach is not based on visions of Christ--although it acknowledges their validity as a conversion experience--so much as on the personal testimony of those most trustworthy men who had witnessed the events of Jesus' life. The variety of models for conversion in the Gospels (see e. g., Matt. 12:38-42; Luke 5:1-11; Luke 8:1-3; Luke 13:1-5; Luke 19:1-10) involve men and some women who change their lives radically to follow the teachings of Jesus, as interpreted by the apostles. These stories take up a remarkable amount of space in the relatively compact narratives of Jesus' ministry. In Mark, there are five such tales (Mark 1:16-20; Mark 1:40-45; Mark 2:13-17; Mark 8:34-9:1; Mark 10:46-52); in Luke, four. These stories come more and more to replace Paul's model for conversion. Even Luke's depiction of the conversion of Paul in Acts fits the new model, not Paul's original prophetic and mystical conception of himself.
Visions of Christ were not wrong in themselves. The apostles had them. But, authority based on visions is always dangerous in religious life, because anyone may have them and claim their knowledge is equal to the leaders', whereas not everyone could claim they sat at Jesus' feet.
Even Luke seems to see Paul's visions as proof of Paul's conversion, not credentials for his apostolate.
The apostolic succession was finite, limited, and definable, as were the doctrines that it produced. Later church leaders had authority because they knew the historical Jesus had transmitted their doctrines and authority to them, not because they had had visions of Christ.
For these apostles, disciples, and evangelists, Paul's faith was too personal, too visionary, and too spiritual. It needed supplementation, in the eyes of the evangelists. Some may have even tried to replace his witness, especially when his thinking was taken up eagerly by the Gnostics. Though Paul's writing was critical for justifying the gentile mission, it was the other apostles whose faith in church organization founded the religion we call Christianity.
Who founded Christianity? If you answered "Jesus," then you get half credit. Certainly Christianity is impossible without the inspiration, life, teaching, passion, and resurrection of Jesus. But Jesus wrote down nothing and gave minimal instructions for how to carry on after him. So who is responsible for the resurrection-focused faith, and who organized the church?