I have tried to explain this to my friends before and I find it useful to draw a similarity with Judaism and Christianity.
Jesus was born a Jew and at some point turned around to the other Jews and said,
"Well, you guys are close, but you've made a couple of mistakes, let me tell you how it really is...."
Gautama was born a hindu, but eventually figured out that everyone else had it a bit wrong and told 'em how its s'posed ta be.