There are two components to learning tolerance and understanding of other religions: the first encompasses open-mindedness, and the second consists of merely acquiring knowledge.
The word "tolerance", especially now in the days of political correctness, is popularly thought to mean "acceptance"; this is not true. In its purest definition, "tolerance" means to "allow" or "tolerate", presumably without discrimination. In other words, one should permit those ubiquitous "other" religions to operate, within the bounds of the law, without attempting to undermine or otherwise interfere with their beliefs.
Being open-minded means that individuals who do not share the same beliefs are willing to learn and accept that others believe differently - but not necessarily agree that those beliefs are true. It doesn't even mean that one who believes differently should acquiesce and agree that others' beliefs are acceptable.
For example, a Christian may "tolerate" the beliefs of a Muslim; the Christian understands that the Muslim has a different viewpoint, and that the Muslim believes he is correct. The Christian's reaction would be "tolerance" of the Muslim belief, but not necessarily agreeing that the belief is correct and true; the Christian may even try to convince the Muslim that he is indeed incorrect. A display of tolerance does not mean that the Christian should not do this, but that he will do it in a way which does not denigrate the Muslim's beliefs or his individual self.
Understanding other religions is, again, a result of knowledge and study. One can study other religions to ascertain the origin of viewpoints and beliefs without compromising his own morals and values. Knowledge and understanding are crucial to developing tolerance.
At the same time, neither tolerance nor understanding are a license for invoking a world religion or combining belief systems or sabotaging established faiths. Neither component should be an excuse to allow detrimental or incorrect or damaging religions to operate within a community. One who is firm in their beliefs should not waver or turn a blind eye to falsehoods in the name of tolerance.
Too often, those who claim to be of one faith, and to live by the tenets of that faith, are woefully adept at turning away from those who pursue mistaken and dangerous religions. They claim they are "tolerant" by allowing those who live by a different moral code to continue their actual immorality and wrongheaded ways. This shows a lack of commitment to their own religion.
Knowledge and tolerance, or familiarity with other faiths and acceptance of the individuals in question, are the keystones to religious equity and forbearance. The question of who is right or wrong is secondary to the main doctrine of most faiths: peace and harmony, a moral code, and striving toward eternity.