The basic idea, still based on the assumption that all lives are equally valuable, is that it is better to save trillions of potential lives in the (far distant) future than billions of lives today. Current thinking about longtermism is set out in the book What We Owe the Future by William MacAskill of the Future of Humanity Institute.There are several things amiss in this passage. First, where do secular tech types get the notion that all lives are equally valuable? Unless one is a Judeo-Christian theist who holds that we are all created in the image of God and loved by Him the idea that we're all equally valuable is nonsense. It certainly can find no support in a Darwinian framework which is the only live alternative to Divine creation available to a secular materialist.
Apparently the longtermists apparently just pull this assumption out of thin air, like a magician pulling a rabbit out of a hat.
Here's another problem with the quoted passage: Why think that we "owe the future anything"? Where does this idea come from? How did we incur the debt? How might a longtermist answer these critical questions?
Lennox goes on to quote a critic of longtermism who wrote that longtermism holds that "humanity has a 'potential' of its own, one that transcends the potential of each individual person, and failing to realize this potential would be ... a moral catastrophe of cosmic proportions."
But what makes it a "moral catastrophe"? Morality is about what's right and what's wrong to do, so why would it be wrong for humanity to fail to realize some vague potential in some far off future? Who would be guilty of this wrong? What does the concept of guilt even mean if, as most secular folks hold, no moral law exists, if there's no accountability for anyone, and if those upon whom this responsibility allegedly falls have been dead for centuries?
The only way any of these assumptions of longtermism could possibly make any sense is if there is a God who imposes upon us a duty to care about promoting the full potential of those not yet born and who holds us accountable for our failure to do so. Yet even those of us who are theists search in vain for any theological indication of who would be responsible and thus accountable for a failure to bring about this state of affairs.
In short, the moral assumptions of longtermism make no sense given a secular or naturalistic worldview, but even if a longtermist accepts the existence of God, those assumptions still don't make sense.
