All religions claim to be the one true faith and all that crap. What makes people think their faith is the right one and all others are wrong? And don't just say "My holy book says so". Every religion has a holy book that says that.
First every religion doesn't have a holy book that says they're the one true religion. The thing that makes one religion the 'true' religion is one's belief in that book. If you're told all your life that Christianity is truth then it will be truth for you. Personally I believe that Christianity is truth and it is truth for me. I can't convince you that it is truth because you have to take that personal step for yourself. So it may not be true for you.
Compare the first century Christians and what their main work was..To preach the good news to all..They went door to door sand preached. They had strict obedience to God and lived the life that was set out for t hem in the Scriptures..They were commanded to preach the good news to all the nations..Can you say that of all So-called Christians??