The more seriously we take our own charge of futility the more we are committed to the implication that reality in the last resort is not futile at all. The defiance of the good atheist hurled at an apparently ruthless and idiotic cosmos is really an unconscious homage to something in or behind the cosmos which he recognises as infinitely valuable and authoritative: for if mercy and justice were really only private whims of his own with no objective and impersonal roots, and if he realised this, he could not go on being indignant. The fact that he arraigns heaven itself for disregarding them means that at some level of his mind he knows they are enthroned in a higher heaven still. *It reminds me of John Gray's Straw Dogs, in which he strives to propose the opposite to Lewis' premise - that man is just an animal, with no awareness of 'something in or behind the cosmos which he recognises as infinitely valuable and authoritative'. Yet at the end we find Gray despairingly admitting that man seems forever doomed to believe, contrary to Gray's perspective, that there is in fact some higher heaven still.
The humanist and atheist will continue to denounce heaven and earth for its futility and thus bear ironic witness to the transcendant ideal of purpose which they deny.
Is that not what the father/editor of Ecclesiastes suggests when he is done reporting Qoheleth's words of futility? (Eccl 12.9-14) He concludes
Fear God and keep his commandments, for this is the whole duty of man. For God will bring every deed into judgment, with every secret thing, whether good or evil.Yet it seems, with the lack of comment he gives to the Preacher's thoughts, that he considers his conclusion drawn from Qoheleth's words rather than in contrast to them. If we find everything vanity, our finding bears witness to the existence of a transcendant standard by which we judge such vanity.
* From 'De Futilitate', in C.S.Lewis Literature, Philosophy and Short Stories [I'm enjoying being back in my parents' home, once more devouring their books until I must leave again!]