"God, say the religious fundamentalists, is the source of all morality; there can be no morality without a Judge who rewards and punishes."
That semicolon contains a big jump. Most people who believe in some higher power do identify that power with good, or morality, or perhaps merely as its source (morality is a gift from God, therefore, without him there would not be good). However, "there can be no morality without a Judge who rewards and punishes" - you seem to define that as to mean that most religious fundamentalists believe that without God, people are incapable of moral behavior. I've heard that applied strictly to the beliefs of some schools of thought within Islam and Calvinism, and loosely used to characterize the beliefs of such diverse figures as William of Ockham and Thomas Malthus.
The strict opinion that a person who does not believe in a judge that rewards and punishes is incapable of moral behavior is very rare among religious believers, fundamentalists (in either the literal or colloquial sense) included. It's really a mistake to apply it as broadly as you are doing when you say "If Overcoming Bias has any religious readers left, I say to you: it may be that you will someday lose your faith: and on that day, you will not lose all sense of moral direction." as if that is what "religious" people largely believe.
More common is the idea, as expressed in the combox here, that lack of religious belief weakens morality. And, furthermore, the aphorism that you took as the starting point for this article "Without God, man cannot be good" the way you are using it is, in my experince, usually bandied about more as one does a theory, in an attempt to explain, for example, the brutality of twentieth century in Nazi Germany, the Soviet Union, Mao's China, Cambodia, etc. Or the relationship between atheism in Europe and suicide.
I've found that hitting either (E) or (I) entails a bit of (W). If you're running regressions on some enormous dataset creating some elasticity estimates, and you're pretty sure that the estimate should be positive and not negative, and you find it's negative you can either hit (E) - systematize the anomalous result: what's driving it and why is this set of datapoints not what the theory would predict - which I suppose is joined by the sentiment toward God that's either (W) God, why the f--- did you make this universe so f----- complicated or (W) thanks be to God for giving the sciences such a vast wealth of material for the highest form of human activity, study (or some other suitable expression of gratitude).
Or you could say (I) - the "bad" option - maybe I'll just try this regression using another specification and forget I ever saw this... and hit (W) to say either thank God no one else saw that or (W) I hope to God no one ever gets their hands on this dataset until it's substantively different...
The empirical evidence using micro datasets that I have read about does dispute much of the theory about religion and morality. Prison populations are much more religious than average. The Southeast has higher crime rates, etc. There's even the aphorism that "there are no atheists in the foxholes." The people who we, as a society, pay to kill people have higher religious service participation rates. On the other hand, charitable giving is perhaps higher among believers. My point there was that overall is that one of the most common usages of the aphorism, in all of my wide reading on the subject, is a highly debatable theory about general trends in human behavior - not a mechanistic theory about human behavior, as the above post implies.