Why are some Christians just so rude in their answers to non believers .?

christianity makes you go through life with an 'us and them' mentality - the "us" being christians and the "them" being the majority of the worlds population that isnt christian. i suppose its understandable that such a mentality would manifest itself in many christians as hostility and rudeness towards anyone who isnt a christian - after all, you either wrongly think ity is an abomination or you dont, theres no room for compromise with christianity