Steve Waldman constructs a taxonomy of the religious left over at Slate. He makes the demographical distinctions seem a lot more confusing than perhaps he has to - he divides the religious left into six different groups when in fact there are probably only two or three with a lot of overlap - but it's an interesting, and sympathetic, analysis nonetheless.
Waldman's six groups are: Bible-thumping liberals, Pious peaceniks, Ethnic churchgoers, Conflicted Catholics, and Religious feminists.
If you read the piece note how many times the implication of what he says about the secular left is that they're little more than unprincipled political opportunists.