Fred Clark, in yet another post lambasting the issues with the Left Behind series, also tackles a more general issue that is reflected in the novels:
This idea of God’s intensely specific individual plan for every detail of your life is a feature of American evangelical piety that we’d be better off without. It’s almost always a source of misery and almost never any help. It burdens Christians with anxiety over decisions that don’t need to be so fraught with moral implication. Choosing a major is hard enough without adding the notion that choosing “wrong” is tantamount to disobeying God.
Obsessing over God’s ISIPFEDOYL also tends to function as a way of distracting ourselves from “the weightier matters of the law: justice and mercy and faith.” If you’re worried about following God’s will, remember that it boils down to this: Love God and love your neighbor. That’s “God’s will for your life.” Take care of that and don’t worry about God’s ISIPFEDOYL.
You can see from that what ISIPFEDOYL stands for: (God’s) intensely specific individual plan for every detail of your life. It is a notion that is also tackled in this image from the blog Stuff Fundies Like: