Be that as it may, what does Witherington have to say about the idea that Christianity was invented by Paul?
One can say that Paul was a catalyst which helped lead the Jesus movement out of Judaism and into being its own religious group. Paul was not the inventor of Christianity, but in some senses he was its midwife, being most responsible for there being a large number of Gentiles entering this sectarian group and not on the basis of becoming Jews first (i.e. having to keep kosher, be circumcised, keeping the Sabbath) which in turn changed the balance of power in the movement everywhere in the empire except in the Holy Land.
In the end, one can say that Paul was a shepherd leading God's people in new directions and through uncharted waters to a new promised land where Jew and Gentile would be united in Christ on the very same basis and with the very same discipleship requirements. Though Paul did not call this end result Christianity, he more than any other of the original apostles was responsible for the birthing of the form of community which was to become the early church. Though he did not invent its doctrines or even its ethics, he most consistently applied its truths until a community that comported with these truths emerged.