Why would anyone belong to a religion he believes is only partially true? I should hope that every person believes his religion is the only true faith and accord everyone else the right to believe the very same thing.
Bingo, but you believe that religion is an expression of reality. To most of today's dim-witted believers, religion is a way of dealing with reality. Even many so-called Christians seem to think that Jesus is an important part of any good self-help program.
Shalom.