Is it OK to "Just Have Faith"?

In an age where we continue to see people led to their deaths by false prophets like Jim Jones, Applewhite, and David Koresh, it behooves us to think very carefully about this problem. Charlatan religious leaders are always going on about “faith” and how we must place our faith in them. Then they kill us or convince us to kill ourselves. And if our deaths don’t interest them, then our wives or daughters, our wealth, our devotion, our opinions, and our independence do! So, it seems prudent to be especially careful about people who tell us to “just have faith.” What is faith, after all? What makes it legitimate vs. deadly?