In fairness to the OP, I don't think that she's really talking about fear, as in the visceral and absolutely soul shattering emotion where you're unable to absorb much meaningful information. It's closer to anxiety, which refers to something that's not immediate and/or not certain. You can still learn when you're anxious about something, at least unless the anxiety is unusually extreme (although making your children anxious when they're around you is pretty messed up, too).
The issue with "learning" when you're anxious is that you're just absorbing the information. You're not able to weigh it rationally in any way, shape, or form. That might be fine when you're in a situation where you're definitely in danger or where only one option out of many is safe and the safe option is obvious, but if the chance of danger seriously needs to be evaluated (or if the one safe route is one that you have to be able to pick out of several possible candidates), then anxiety can put you at more risk. It can make you accept things that are patently irrational.
It's inappropriate when applied to something like Christianity because you have to be able to actually analyze, rationally, what the risks and rewards associated with belief or non-belief are and the likelihood of each of those happening. Anxiety overplays risks and their likelihood.