There's some interesting commentary here
on the signifigance and usefulness of having friends who are outside of your own faith, whether that means friends who are Jews, Muslims, or atheists/agnostics.
I would say I haven't had quite as much experience like this as this writer has, but I do count several Muslims and atheists friend, as well as people from many different Christian faiths (here the Internet has been an invaluable asset to my faith). I agree with his conclusion: having theological discussions with these people has deepened and strengthened my own faith, often by knocking it down. That's painful, but that's what I need. Friends who believe as I do are a comfortable sanctuary, but they tend to reinforce me where I'm already strong, while friends who are on the "outside" help me grow where I'm weak. If nothing else, they help you to understand that a great deal of the world does not think as you do (regardless of your particular faith), and if you want to reach those people, you must understand them. And you can't understand those that you never talk to.