Lack of doubt:
....If we assume that absolute truth exists in theological matters, then the chances of one religion picked at random having the whole "truth" is much less than 1%. Yet, for many people, religion is an accident of birth. .... A person born in Saudi Arabia will almost certainly be a Muslim in adulthood; a person born in Alabama will most probably be a Fundamentalist or other Evangelical Christian, and so on.
Top-down vs. bottom-up religions:
Among monotheistic religions: Top-down religions are grounded on a deity revealing truth to humanity, typically through prophets. ... Many, perhaps most, people regard their own religion as the only top-down faith in existence. Being revealed by God, it must be true. Theholy book is God's word. All of the other religions in the world are seen as bottom-up faiths, and are thus severely limited by cultural factors, lack of knowledge of the social and natural sciences, etc. Given that their own religion is the only one revealed by God, some find it difficult to tolerate other religions who only have part of the truth, particularly when they "know" that they have the full truth. ....
The existence of Hell:
..... Many, perhaps most, people believe that they and their fellow believers will end up in Heaven, and that most or all followers of other religions will go to Hell. That is, God has such a low opinion of other religions that he will have their followers tortured for all eternity without hope of mercy or relief. ...
In religion, dualism is the "concept that the world is ruled by the antagonistic forces of good and evil." ....
Theory of Reciprocity:
This is sometimes called the "Golden Rule:" to do onto others as you would have them do onto you. All of the major religions have their own version. Unfortunately, organized religions seem to have miscommunicated their Theory of Reciprocity to their own members. The "do onto others" is supposed to refer to all human beings. Many religious folk interpret this to mean only their fellow believers. In one of his most famous parables, Jesus talks about the Good Samaritan who helped a victim who was not of his religion. Jesus defined one's neighbor to include the entire human race.Linking religion and nationalism: Many people have closely linked their religion and their nationality. We see this in the quotation by George H.W. Bush above where he denied citizenship and patriot status to those who do not believe in the existence of his God....
This is the concept that the responsibility for the actions of one person or a small group of people can be transferred to all humans who happen to belong to the same race, religion, gender, or sexual orientation, etc. And so, some people held all Muslims -- and people who merely looked as if they might be Muslims -- equally responsible for the terrorist attack on 9/11 by 19 real perpetrators.
Responsibility across generations:
This is a variation of collective responsibility where an entire group -- a race, religion, gender, sexual orientation etc. -- is considered responsible for the actions of one person. Here, an entire group is held responsible for an event that happened generations, centuries or even millennia ago. Until the mid-20th century, some Christian denominations taught that every modern-day Jew bears responsibility for the execution of Jesus by the occupying Roman Army in the1st century CE. This happened almost two millennia ago.
Fear of the other:
Humans seem programmed to be cautious towards people who are different from themselves. ..... Fear can lead to rejection and worse.
Passages in their holy text:
Many passages in the Hebrew Scriptures, Christian Scriptures and Qur'an call for active discrimination against members of other religions. Some parts of the Bible describe murder, mass murder and genocide for religious minorities. A believer who merely follows their holy book can find lots of justification to engage in religious discrimination today. Fortunately, these holy books also have passages calling for religious tolerance. It all depends on which passages are given the most attention.
Not too many decades ago, it was common for Protestants to be instructed to not enter a Catholic cathedral, and vice-versa. Some Protestant pastors are required by their churches to never attend inter-faith gatherings. In the past, many faith groups taught that only their members would attain Heaven and that the followers of other denominations in the same religion, and followers of other religions, would all go to Hell. Hatred and animosity by some believers are thus merely extensions of church teacings. Fortunately, such teachings have since faded.