Members of one faith criticize another because they either believe their path is the only way and they are not tolerent of others or they were taught by their parents.
People know their path is right because they've seen evidence of it or because that's what their parents taught them. Sometimes they just like the concepts behind the religion.
In my opinion, Jesus is the only way to god. And that anyone can go to Jesus. I don't criticize other peoples religions. I just question them or state my opinion on why i don't believe in their concept. I know my path is right because there are many facts to back it up.