There are billions of people. Most of them hold beliefs that are different from each other. For all these people to co-exist, it is important that we learn how to respect each other's beliefs without feeling threatened by them. To do this, we must first recognize the difference between belief and truth.
First, please read my definition of Truth and Belief. This what I mean when I use the terms truth and belief.
The most important difference between truth and belief is that all truths are eqaully true. They are, by definition, absolutely true. Beliefs, on the other hand, are not proven, therefore some beliefs may be more likely to be true than others. There can be evidence to support beliefs and evidence to disprove beliefs. This means that all beliefs should not be treated as equal. Belief without evidence is often refered to as faith.
Most people have beliefs that they treat as truths. An easy example of this is God. Many people believe absolutely that a God exists and some people believe that God does not exist. However it is impossible to prove the existance of a God without the God's assistance. Equally, it is impossible to disprove the existance of God. The God example is a bit extreme, but it is also very universal.
As beliefs are not proven, and in some cases are not even provable, one must be willing to accept the possibility that one's beliefs are incorrect. If one accepts that personal beliefs could be wrong it allows one to entertain the idea that other people's beliefs might be correct. (It is important to note that one does not need to adopt another's beliefs to understand them). The ability to hold another's beliefs a equal to one's own, it the first step toward respecting them.
It should also be noted that these ideas are not restricted to religious beliefs. There many examples from history of long held scientific beliefs being overturned by new ones. The concept of Aether is a good example. Another might be Geocentrism verses Heliocentrism verses the modern view of the Solar System. However within science, beliefs are based on evidence (as opposed to faith). This means that as new evidence is discovered, old beliefs may need to be changed. This is, perhaps, the very definition of science. (Read by page on Science for more information).
If one treats a belief as a truth, then it is much easier to be close minded about conflicting ideas. Two individuals with conflicting beliefs (treated as truth) will have a far more difficult time understanding each other. Further, the differences can more easily be used to divide people into groups. I believe that dividing people into groups make it far easier for those groups to mistreat each other. For example most Theocracies (or perhaps more correctly Ecclesiocracies) enforce their beliefs on to people. To believe differently is equivalent to being a criminal.