If faith is what we must have in order to be saved, then we must have doubt as well. The famous opening of Hebrews 11—Ἔστιν δὲ πίστις ἐλπιζομένων ὑπόστασις, πραγμάτων ἔλεγχος οὐ βλεπομένων"—is important. Faith is the assurance (hypostatis) that what we hope for is in fact real, the conviction (elexos) of what isn't seen.

This clearly calls for self-examination, because it implicitly states that faith is not certainty.

Now I am certain that if I should drop my keys, they will fall toward the center of the earth of the rate of 32 feet or 9.8 meters per second squared, unless they hit something, like a floor or the ground. I have personally verified that this is so during my first year of physics. However, my faith cannot be summed up by an equation. And since this faith isn't certain, then there has to be from time to time some doubt.

This is what the Bible teaches, as you know, Dan. God will allow our faith to be tested, like a refiner's fire. Should a preacher or even an angel claim that certainty is needed instead of living faith, let them be anathema (Gal. 1: 8).