Atheist Roundtable

Every time I talk to a believer these days, I hear two things:  the believer thinks most christians are wrong and those christans are not following the bible; and the believer to whom I am speaking is right and IS following the bible.   What's us with the no-true Scotsman nonsense?  Why is this used so damned often?  How can atheists respond to it? My thoughts plus your calls equal awesomeness.