It is very important to understand the other religions in the world, because when you are talking to someone about your beliefs, you should know the back ground of his or her religion.
The largest religion in the world is Christianity. The second largest is Islam.
Even if you are not a Muslim, you should learn at least the basics of Islam. I am definitely not a Muslim, and I am not saying that you should be a Muslim, it is just that so much of the world news has something to do with countries that are Islamic and you should know where those countries are coming from with their policies, moral standards, etc.