I think Paul was inspired by the words of Jesus, and the changes that Christianity made were as a result of trying to conform to their experience of Jesus. That is, the changes that Jesus' instituted that later became church doctrines required a new conceptual scheme to be instituted among believers. Hence, as the new conceptual scheme evolved the religion settled in a particular basin of attraction which is where orthodoxy had finally arrived. Paul had a huge impact on the basin that Christianity arrived, but I don't think what Paul did was inconsistent to the overall message that Jesus preached (i.e., the intent was maintained).
As for combining religions, I suppose that is happening whether we like it or not. The issue, though, is how it happens we really have no control over. The events that happen in the world will ultimately decide whose beliefs have a greater weight than other beliefs. No one can predict which events will happen, and how they will affect future religious beliefs. If, for example, we encounter extraterrestials in the next century who have a religion very similar to Jainism, then the next century I would bet heavily on Jainism becoming a major religion that overtakes all the others. We just don't know where the future will take us.
Therefore, we shouldn't try to predict the future. Nor should we try to unite our beliefs with other religions in my view, rather we should sit back and enjoy the religion that touches us the most (given our particular conceptual schemes) and share that view in a tolerant and respectfull matter. Let the future take care of itself. The present is what we should relish.
Warm regards, Harv