At the end of his previous book, How to Stop Living and Start Worrying, Simon Critchley wrote: "If morality becomes a question, as it is on BBC Radio 4, of nicely educated people with shrill voices making choices between different courses of action and being able to account for them, then this is awful." With The Faith of the Faithless, Critchley shows what a moral philosophy that goes beyond media bloviating might look like, and moves his own thinking beyond the traumatic aporia with which How to Stop Living and Start Worrying ended. Critchley was one of the first thinkers to stress that so-called "continental philosophy" had an ethical dimension over and above its worrying and fraying at the textures of language, and that manner of analysis typifies this new, movingly optimistic, work.

In Infinitely Demanding, Critchley had already begun to articulate the methods by which the state of being a "dividual"  "the self which shapes itself in relation to the experience of an overwhelming, infinite demand that divides it from itself"  might represent the precondition for committed engagement rather than a paralysing paradox. The Faith of the Faithless outlines in more detail his views on the nature of conscience and the possibility of an "anarchism of responsibility".

It does so through close and nuanced readings of Rousseau, Heidegger, Oscar Wilde, St Paul and Kierkegaard. Along the way, he has some pointed and precise things to say about two other high-profile contemporary philosophers, John Gray, and Slavoj iek, over whose review of Critchley's Infinitely Demanding there was a testy public debate. (iek used the review to fulminate about anti-war protesters as tacitly providing support for the war: the state could say "see how we allow demonstrations against us" while the protesters could not bring themselves to deploy the "divine violence" that would effect revolutionary transformation.)

The chapters of this new book do not establish and develop an argument. Instead, they parry and complement each other; it is better to think of them as symphonic movements. The theme on which Critchley plays his variations is established in the introduction: "Rather than seeing modernity in terms of a process of secularisation, I will claim that the history of political forms can best be viewed as a series of metamorphoses of sacralisation." Wilde's idea of a "confraternity of the Faithless  where on an altar, on which no taper burned, a priest, in whose heart peace had no dwelling, might celebrate with unblessed bread and a chalice empty of wine" motivates the interlinking ideas: how can those who "cannot believe" reclaim from the vestiges of religion the moral authority and political radicalism that it once had? How can Wilde's italicised "cannot" stand upright? "Everything to be true must become a religion," Wilde says, and Critchley, poetically and persuasively, suggests ways in which this might be accomplished. Yet he seems racked by doubt on whether it ever will.

The first variation is on Rousseau, and his letter to Voltaire asking  or demanding: Rousseau is always the sly hysteric  that Voltaire write a "catechism of the citizen". How could secular societies find a binding mechanism that effectively replaced traditional forms of unity? Critchley doesn't raise the spectre of Robespierre's cult of the Supreme Being, the practical (and failed) application of Rousseau's ideal.

The second chapter carefully unpicks the persistence of original sin in the thinking of Carl Schmitt and John Gray, whose "passive nihilism"  a profound pessimism that we "killer apes" can ever improve ourselves or the world, leads him to cling to the aesthetic as the only consolation in this vale of tears.

In his penultimate experiment Critchley turns to St Paul and the idea of faith, with particular reference to Heidegger's references to Paul. And Crichley's account of the "Pauline turn" in the work of Agamben, Badiou and Taubes is astonishing. But he relies a little too much on the figures he is deconstructing, rather than on Paul himself. In the first and second letters to the Corinthians, the self-abasing Paul is just as Critchley depicts him: a man suffering Critchley's infinite demand. But, historically, by the time Paul is imprisoned and writing to the Ephesians and in his so-called "pastoral letters", a different Paul emerges; a braver Paul, a man who has somehow discovered confidence. Whence this power? Whence this bravery?

The final chapter is the most rebarbative, and the funniest. Critchley is obviously peeved at iek, and, rather than simply opposing his beliefs, puts him on the couch instead. Coyly claiming to "depolemicise" the debate, he turns iek into a teenager, who sits by idly while fantasising about smashing up either the state or the local Tesco. Critchley writes in sorrow: while being the best proponent of hope, he is stricken with hopelessness; while imaging a better tomorrow, he can't help but remember yesterdays. He is moving when he describes "the faithless who can best sustain the rigour of faith without requiring security, guarantees or rewards". But believers, as well as non-believers, are no longer allowed the naivety of a pre-Kierkegaardian faith. As the old hymn goes, the difference is in the possibility of "love to the loveless shown / that they might lovely be".

 Stuart Kelly's The Book of Lost Books is published by Polygon.