Belief that one's mother is a cockroach is delusional. It can be proven to be false. Belief that one's mother is a reincarnation of an entity which was a cockroach in a previous life is not delusional. It can't be proven to be true, but it can't be proven to be false either.

This isn't really the point. What is delusional about religion/faith/God is not that you believe in it but that you act, or live your life, according to your belief in it (and, worse, compel others, such as your children, to act accordingly). By any measure of rationality, the likelihood of God's existing is vanishingly small. I'll spare you my three-point argument for that as I've parroted it out a number of times already on those other threads. It is therefore rational to live your life in spite of what others believe about him. It is not rational to live your life according to a notion that is exceptionally-highly improbable. As for mother being a cockroach, the dedicated believer would argue that you can't prove she isn't one because he sees a cockroach shape where you see a woman shape. Believers always have last resorts of this kind. The belief that she was cockroach in a previous existence is slightly different to a belief in God because you can hardly live your whole life according to that belief. There isn't much you can do to modify your behaviour as a result of it, unlike the belief in God. Though again, you could put up a very good argument for the probability of her once having been a cockroach to be vanishingly small, even though you can't prove she wasn't. Let's say that on Dawkins' belief scale, on which he places himself as 6.9 out of 7 on the non-belief scale, you could plausibly say that you believe mother was never a cockroach, score 6.9. And you believe that the ex-cockroach notion is still a non-delusional belief. I don't really think I agree.