"Any atheist who says "delusion" of belief in God is dealing in certainty. Delusion implies that you know it is wrong."

In that case we need to excise the word from the dictionary. There can clearly never be a suitable context for its use. Or we can continue to use it for people who fiercely hold to beliefs that have a vanishingly-small chance of being true. If I told you and everyone else that little blue men with seven legs and sixteen eyes skated on the rings of Saturn every Friday, preached it from pulpits and handed it down to the kids you would call me deluded (and a few other things). But, of course, it might just be true...