Someone I know from academia is supporting to his students (one of whom I know) the idea that Jesus only intended to reform Judaism, and that Paul is the one who really got Christianity as such going. I know this is wrong (“on this Rock I will build my Church” for instance), but need to be somewhat more academically sophisticated for my young friend.
Yes and no. The Catholic church is the fulfillment of the Old Covenant. As such, Christianity makes up the completed Jews. We are God’s chosen people (as the Jews still remain to this day). So in a way the teacher is both correct and incorrect since Catholics remain “reformed” jews to this very day.
If your friend is getting into the so-called “historical Jesus,” I’d recommend reading some of the scholars who actually use the historical-critical method in a responsible way, e.g., N.T. Wright comes to mind. Pope Benedict has also written some very scholarly works, e.g., Behold the Pierced One or even his latest book.
It’s tough to provide a blanket refutation since this is a very broad topic. In general, I’d keep the following in mind: Not that much time elapsed between Jesus and Paul, Paul shows clear evidence of holding to a pre-existing tradition, none of the apostles acts as if Paul is doing something completely new. There are really a lot of points you could make: Jesus wouldn’t have been crucified for trying to reform the temple, the very early Christians believed in the Resurrection, worshiped Jesus as God, etc. and had to get this idea from somewhere.
Good luck and keep us posted!