Even if we live in a society in which everyone is of the same religion, it is a small world, and it is inevitable that we will encounter people of other religions. To function in the world today, tolerance should be a practical imperative, for our own well-being, it should be a moral imperative, and in the United States and most other western nations, it is a constitutional imperative.
We are living in a time of global trading and mass immigration. There is no value in religious intolerance under these circumstances. For example, the United States is actively courting India as a trading partner. If we choose to be religiously intolerant of Hindus, this is not likely to be a successful courtship. The United States does trade extensively with Mexico and other Latin American nations. These are largely Catholic countries, while the majority of Christians in the United States are some form of Protestant. If we choose to be intolerant of them or they choose to be intolerant of us, this will not work out very well. As far as immigrants are concerned, several western nations have just committed to taking in thousands of immigrants fleeing Syria and other Middle Eastern countries. If we inquire as to their religions, even aside from the ethical issues of allowing people to languish and die, we cannot possibly assimilate them successfully by starting out worrying about their religion. So, from a purely practical point of view, tolerance gets everyone far further than intolerance.
Religious tolerance really should be a moral imperative as well, since intolerance is so very bad for our character. And current horrors propagated by radical fundamentalists notwithstanding, pretty much every world religion at least preaches tolerance. Religion is a matter of faith, of belief, not of evidence, and if we do not have respect for the beliefs of others, it is to our detriment, since a lack of respect diminishes us as human beings. Everyone is searching for an answer, and why should anyone be so egotistical and solipsistic as to think he has found the only true way? If one believes in a deity, one should be aware there can be many paths to that deity. Furthermore, we can hardly expect others to have respect for our beliefs, too, if we are not tolerant, and then the intolerance is simply compounded. Intolerance is evidence, in my opinion, of flawed character and morality.
In many countries religious tolerance is actually codified. In the United States, it is in the Constitution, in the First Amendment. No, you will not see the word tolerance mentioned anywhere in the amendment. What you will see is that the government may not choose to promote one religion on behalf of the government. That is the establishment clause. We can infer from the establishment clause that we demand that our government be tolerant of all religions. In the free exercise clause, we insist that everyone be able to freely exercise the religion of his or her choice. Again, we can infer that we are expected to tolerate one another's religions, since preventing someone from exercising his or her religion is surely intolerance. In fact, our entire existence as a nation is a result of people who had experienced religious intolerance in England, seeking a new land where they could freely practice their religion. This is a dearly held American value, as it is a value in many other nations, too.
In spite of the ugliness we see around us today, ugliness caused by religious intolerance, responding with intolerance is not the answer. If we want to get along in the world, if we want to be of good character, and if we want to value what America and many other nations hold as dear, we should all be practicing religious tolerance.