A newspaper article that recently showed up (I'm told it was from the Houston Chronicle, but I can't find it there anymore; this guy seems to have the article) asked what the "most dangerous idea in religion today" was, and gave the responses of five rather famous religious leaders, including the likes of Deepak Chopra and Rabbi Harold Kushner.
With the exception of Richard Land, all of the answers centered around the idea of extreme tolerance; it's dangerous to tell people their behavior is wrong, their religion is wrong, or attempting to convert anyone to your own faith. In other words, the underlying assumption in all of these is that there are many paths to God, none of them wrong, but attempting to claim that yours is the only path is itself wrong.
I find the premise of the article amusing, because this "today" thing is a bit of a stretch. Really, there's nothing new under the sun philosophically speaking, and that's especially true of religions founded millenia ago. Still, my own take? I'd have to give two answers.
If you're asking which idea is most dangerous physically, I'd agree with Dr. Land that violence in the name of God threatens us the most. There's a strong resurgence in the world of Islamic Imperialism, and its adherents are willing to go to any lengths to attack those they see as standing in the way of that. Whether that means gunning down a street full of civilians or suicide bombing a group of soldiers, these guys seek a theocracy by any means. Even more frightening, many in the world have become too frightened of appearing "intolerant" or "racist" to face the problem as it presents itself and opt to hide behind the skirts of a ticking clock. This problem isn't critical mass now, they must be reasoning, so I can get away with pushing it off to someone else to deal with. This vulnerability makes the problem far worse.
However, the most dangerous idea spiritually, the one most likely to hurt people when they face their maker, would be the very idea the religious leaders were defending, that there are many ways to God, that any and all expressions of religious belief, as long as they are sincere, will merit a man heaven.
I find such ideas to be a comforting thought if you'd rather not be serious about the matter, but reality is harsh. Some ideas are better than others, and when you have religious systems that present reality in ways fundamentally different from each other, both cannot be correct. Judaism teaches that Christians worship a false messiah . . . how could both possibly be on the same path to God? Islam teaches that Christians and Jews altered their scriptures and deny the truth Muhammad brought, which consigns them to Hell. How could these ideas all be leading to the same God?
I open the question up to you, readers. What do you consider the most dangerous idea in religion?