There is a misunderstanding happening here. @Patrick is correct that an atheistic world can have purpose, in that humans can find purpose and meaning on their own. That is certainly correct. @Marty is pointing out that this type of purpose seems to be fragile, and ultimately we can be certainly is goes away (such as when we all die or when our purposes are frustrated). That also seems true.
The existential question is whether or not there is any purpose in the universe that transcends us as humans, and if matters. @Patrick, I am sure, would say no on both counts, and @Marty would say yes. I think this debate is clarified in the context of clear violations and contradictions of humanistic purpose (such as genocide). When humans construct purpose their own, we can never be certain such purposes will be ultimately good or achieved.
Contrast that with what MLK says (The Arc of the Moral Universe Is Long, But It Bends Toward Justice – Quote Investigator®):
The arc of the moral universe is long, but it bends toward justice.
He is quoting a long theological tradition:
“We must believe that the arc of the universe is long, but that it bends toward justice, toward one Divine end towards which creation moves onward and onward, forever.”
And:
We cannot understand the moral Universe. The arc is a long one, and our eyes reach but a little way; we cannot calculate the curve and complete the figure by the experience of sight; but we can divine it by conscience, and we surely know that it bends toward justice. Justice will not fail, though wickedness appears strong, and has on its side the armies and thrones of power, the riches and the glory of the world, and though poor men crouch down in despair. Justice will not fail and perish out from the world of men, nor will what is really wrong and contrary to God’s real law of justice continually endure.
Any notions we have of being on the “right side” of history, or conceptions that ground the type of voice we have in MLK, can’t really be justified in secular humanistic point of view (or at least I can’t see how).
Sure, the idealism exists, as we are all gear to talk about utopias. In practice, these utopias quickly slide to dystopia. We could just as easily argue that the arc bends towards injustice, and perhaps the winners are the oppressors. In fact, that is exactly what many do, and they seem to be very successful.
This, I think, is the sort of thing that should give us pause to dismissing questions about transcendant purpose. Without some normative purpose beyond that which humans create, there is no clear grounding (other than preference) to denigrate one man’s decision to find his purpose to genocidally end another community so that his progeny can rule the earth.
Without doubt, @patrick is aligned with us against such evil, however, I’m not clear the resources his conception of the world brings to the table to push back against such evil. We should have no confidence that evil people might respond to the call to goodness. We are left with a power struggle, without a way to even know what is good and what is evil. Of note, that is very much what society looks like now too. So, in one sense, @Patrick’s view is very coherent with the world. It does seems that right and wrong are ambiguous, and that power rules the day, so we should amass power and use it. After all, if coercive power is the rule, it is better to hold that power and either use it for ourselves, or (if we are more moral) at least prevent others from using it coercively. That makes some sense until society as a whole choses together to do evil. We have certainly seen this time and time again.
From @mary and my point of view, we see a hidden order, that can emerge. The world seems unjust, but could be just. Injustice is not merely a human construct, and our relationship to justice and injustice is not indeterminate. Our purpose is to be a just society. Society, however, is not just. Just like MLK, we believe that the Day of the Lord is coming, where God will rebalance the world even for those who died. In the meantime, we believe (or at least we should believe) He works with us now to bring forth a new sort of Kingdom on Earth, one that is just and redemptive precisely because it is not merely our construction. Likewise God made us, even those who turn to evil, such that we can respond to the call to justice.
We believe even racists are redeemable. The world is not just, but God made all of us, including racists, to thrive in a just world. This, it seems to me, is a critical part of MLK’s message that makes it so coherent and attractive, even to those that do not believe in God.