When is a Religious Belief Delusional?

[Whateverist]

nevertheless, there seems to be an overarching concept of human goodness that unites them all.

If by "unites them all" you mean only that the concept applies in all cases, perhaps there is. That is a hypothesis that could be checked.

From there it is not too difficult to derive an overarching concept of goodness shared by both humans and non-human species. And from there it is not too difficult to conclude that there something like ‘The Good’ in which all good things participate and that this transcendent Good is real, in some ways perhaps more real than everyday reality.

No, the only claim to reality available to a concept is the accuracy with which it reflects the phenomenon to which it pertains. A concept isn't so much real in its own right as it is true. Perhaps you believe in a realm of Platonic archetypes whose ultimate reality exceeds the actual instances we find in the world. If you do we aren't using the words in the same way.

This is by no means a clear-cut derivation but it was the conclusion made by Plato and adopted by many later pagan and Christian philosophers who call this God. They may indeed be mistaken. Most AF member seem to attribute the moral sense to some combination of animal instinct and social pragmatism. This is merely a difference of opinion between Plato. Plotinus, and Aquinas, among others and AF members. I see no justification for piling derision on people who hold opinions similar to some of the greatest thinkers of the past.

Some of the greatest, mistaken thinkers of the past perhaps.