Most people think that being moral means acting in a way that will earn you credit with God.

That, but I think there's some reciprocal after-effects that also come into play. What I mean is that when you view what being moral implies with respect to one's religion, you get what you suggested -- being moral entails an increase in heaven (or whatever) being likely.

A very interesting effect I've noticed going the other way, is that religion lets you discuss morality in far, far, far more "lofty" terms that what a non-theistic individual might come up with. The "worldly" discussions are about utility, catagorical imperatives, maxims, means/ends, etc... but "common-talk" religious morality involves "being Christ-like," "being a light to others," "showing them Christ's love," "being a witness," "acting as a suffering servant" and the like.

These just plain sound amazingly magical while [to a religious person especially] the other discussion about morality can sound cold and calculating. It reinforces the notion of doing something supremely fantastic in one's quest to be "moral" via the religious lens.

Now, I think all of these basically translate to:

act so amazingly awesome according to social standards that my observer will think that the only possible way to be thaaaat good is if god himself is infusing me with supernatural spirit powers to do so; thus... god exists and you should convert

But it's an interesting point not unrelated to your theory.