First of all, it is illogical to say that all religions are true. That is like saying all colors are blue, or all people are white. Every religion cannot possibly be true. It is more logical to say that all religions are wrong, than all religions are right. Hinduism is greatly flawed in this assumption.
Islam and Christianity both believe they are true. They want to convert the whole world because they believe that they are helping people with their eternal destiny. However, the way both of them present their messages are radically different.
Radical Islam converts people by force, at gunpoint, by threats. It is a religion of laws. Do this, say this, act this way, and Allah will bless you.
Christianity, however, does not 'force' people. Christians try to persuade folks to believe that Jesus is the only way to heaven. Their faith is radically different, because it is a religion of grace. God saves us, we don't save ourselves. We can't work hard enough or say enough prayers or bow enough or cut ourselves enough, or do penance enough to earn salvation. All we are required to do is believe. God does the rest. It is by far the easiest way to get to heaven, yet so many refuse to give it a chance.
The difference with Christianity and all other religions, is that Christianity is the only one that has a man who came back to life from the dead. Jesus was resurrected. He is still alive. No other religion can say that. Since He told us He was the ONLY way to heaven, should we not believe Him? There were over 500 witnesses to His resurrection! When you know something to be absolutely true, you want everyone else to know it. Wouldn't it be wrong of us to keep this to ourselves, when we know that it is absolute truth? How would you feel if you found out Christianity was the only way to heaven, and we all refused to tell you about it, and you ended up in hell? We would be doing you a great disfavor. Christianity's only goal is to insure that you come to know the best friend you'll ever have, and have eternal life in heaven.
In Islam, Allah is NOT your friend. I sure don't want to spend eternity with a god who is unreachable and hard to talk to.