Every religion was founded on one basic principle: to feed, clothe, house, transport, and (officially in the more liberal religions, unofficially in the rest) sexually gratify the shamans, priests, ministers, imams, or prophets of the religion, without requiring the shamans, priests, ministers, imams, or prophets of the religion to do any real work in exchange. All other principles were added later, to flesh out the religion and make the people think that they were actually doing something real, instead of just paying for the shamans, priests, ministers, imams, or prophets to have an indoor job with no heavy lifting (other than big bags of money). As such, if someone comes along saying "hey, stop paying for *your* cleric's religion gravy train and start paying for *my* cleric's religion gravy train", naturally the shamans, priests, ministers, imams, or prophets of the current religion are going to command their flock to slaughter this "heretic". Especially if the new religion doesn't pay for the support of its clerics (or even have the option of doing so), the Ultimate Heresy in mainstream religion. And heaven forbid you are an Atheist. Even the "we don't pay our clerics" religions still follow the commands of their unpaid religious leaders, but the Atheists don't even accept the concept of a "religious" leader. Both the "gravy train" and the poverty-stricken religions can hate the Atheist viewpoint.

All religion is founded on the concept that it is singularly and solely right in everything it preaches. When you believe that you're absolutely right and everyone else is wrong, and that there will be serious consequences like "Damnation" or even "The Wrath of God" for those who disagree with you, how tolerant are you going to be of those "Wrong" people? Some level of intolerance is naturally bred into religion, and religious people who are completely tolerant of others' beliefs are usually only doing so by going against their religion.

For some reason when people are a part of a group they have the tendency to feel like they are part of a team. People want to beleive that their religion/ team is the best and they end up doing cruel things to show it. Human Nature. Also, when you are a believer of a religion you will tend to beleive that everyone who beleives things other than what you beleive are wrong, therefore you develop a superiority complex and that also leads to the dissention between you and people of other faiths.

Because religion is mans attempt at pleasing "god", and earning points to make it to their version of heaven, based on their religion. Mankind will never be able to tolerate another "religion" other than their own because man is a self- centered, egomaniac when left to his own devices. History proves this to be true. The only way anyone can truely love another, or do right to another, and get along with another, is to give up their "religions" and have a relationship with the one who is love.