The problem with rejecting the scenario provided is that it happens all the time. Lay-persons, preachers, theologians, etc. sometimes "lose their faith". Are we to say then they were never saved? If so (i.e. if we say they were never saved), then what assurance do we have? Because some people who are very sincere in their profession of the true faith (i.e. they aren't lying)do, in fact, stop believing.

Grace is not common.