 They tell you that all religions are supposed to be respected and that ultimately all religions lead to the same God. That is, of course, a contradiction that cannot be if you actually look at the theology of those religions. Judaism proclaims that Jesus is not a prophet. Islam, on the other hand, does. Christianity goes so far to say that Jesus is actually not only a prophet, no. He is God indeed. So those three belief systems cannot coexist. It is absolutely impossible. It goes against simple logic and the law of non-contradiction. Jesus cannot be God and not God at the same time, which then implies, of course, that Christianity cannot be true if Islam is true.