Matt Powell of Wheat and Chaff writes:
In MLK's cosmology, and in mine, all men are equal because all men have souls created in the image of God. That is our source of equality. Take away the soul, and the equality is gone.
As I understand the rest of his post, Matt seems to be asking for some ability or descriptive/substantive attribute that all people have in equal proportions. But people vary according to just about any measure one might care to imagine. So he suggests we all have equal 'souls'. I'm not sure what this is supposed to mean. Perhaps the 'soul' is our innermost self; the part of us that makes difficult decisions, engages in moral deliberation, etc.? If that's the case, then it just doesn't seem true that our souls are any more 'equal' (in the descriptive sense Matt is after) than the rest of us. Just like some people are faster runners than others, so some make better decisions, are more morally developed, and so forth. So if 'soul' means anything like what I suggested above, it cannot do the work Matt demands from it. (And if it doesn't mean this, then I'm not sure it means anything substantive.)
But of course, when we talk about people being 'created equal', we're not talking about any such descriptive equality. Rather, it is meant as an affirmation of moral equality. This might be best understood not as a substantive property possessed by others, but rather a claim about how we ought to treat them. Everyone is (prima facie) worthy of equal consideration. It would be wrong to discount someone else's interests just because they're of a different race or religion from you. More succinctly: all count in the moral calculus.
Although very abstract, I think it's a simple enough concept for any moral agent to understand. It's not about how fast we can run, how rationally we can think, or any other ability or descriptive property we may possess. It doesn't require that we have some internal organ that is literally identical or 'equal' to our neighbour's one. So it doesn't require God-given 'souls' - indeed, it doesn't require religion at all. It's simply about morality, and how we ought to treat others.
[O]n what basis do you believe men to be equal? And what scientific evidence could I provide you with to prove to you that different races of people might in fact be unequal?
If there is no scientific evidence that I could provide you with to make that case, then that sounds a lot like religion to me.
Matt seems to have confused religion with a priori knowledge generally. I can't imagine any scientific evidence that would 'prove' to me that not all bachelors are unmarried, or that 2+2=5; does it follow that my mathematical and semantic beliefs are somehow "religious" in nature?
To answer the first question: some reasons for including all people's interests in the moral calculus were described in my recent post, objective moral relativism.
As for whether this claim is revisable, I have my doubts. I think the fundamentals of morality are a priori, though of course the details depend upon contingent facts. So while scientific findings may show how to better help people (and thereby change our particular moral beliefs), it's difficult to see how it could cast doubt on something as axiomatic as moral equality. (Though scientific advances have been known to cause revisions to what beforehand seemed like analytic truths - so who knows for sure?)
But we could, at least, speculate as to possible circumstances which might downplay the practical import of this principle. Imagine the 'Lucky' race, whose members tend to be very happy and satisfied almost independently of contingent life events. Since nothing we do is likely to thwart their desires anyhow, we might well employ the heuristic of ignoring the interests of Luckies. (Within limits, of course; death would obviously still harm them!) We might treat other races as if they were more important, morally speaking. But this would not be a case where moral equality was genuinely 'disproved'; in actual fact Luckies are still just as important as anyone else. It's just that, as a practical matter, the best way to fulfill (everyone's) desires generally is to ignore Luckies and put more energy into looking after everyone else.
Update: Matt responds here.