Having irritated atheists last time, I now want to turn to Christians. The vast majority of them, including the most highly regarded theologians, still resist Rousseau's basic insight concerning religion and the state. It is this: the state needs common values that its citizens can agree on (what in The Social Contract he calls a "civil religion"). Christianity, in its traditional forms, cannot be this civil religion, as opinion is so divided over its doctrines, and because there are many who do not believe in it at all. The modern state rightly moves away from traditional religion as its source of ideological unity, in favour of post-religious humanism (equality, rights). Christians should accept, indeed affirm, this shift, seeing secular humanism as an indirect expression of Christian values. Instead, conservative theologians (including postmodern "radical orthodox" ones) think it brave to oppose secular humanism, to paint it as a threat. This only makes communication of the gospel harder.

But, of course, Christians cannot simply affirm secular humanism (they wouldn't still be Christians if they did). They should also warn that it has a deficient understanding of human life; part of this is its implied belief in natural human goodness. Rousseau is rightly seen as a key exponent of this belief. He adapts the Christian idea that humanity is made in the image of God and is therefore good, detaching it from the wider narrative of human fallibility and dependence on God's grace.

Of course, he does not entirely deny human fallibility, error and capacity for evil. But he treats it as inessential: something that can be understood and moved away from – through trust in the wisdom of the human heart.

And he offers his own life as a model of this therapeutic process – supremely in his Confessions. It's the first really modern autobiography: Rousseau's fascinated focus on his own psychological development is unprecedented. (By the way, it's a book of two halves: the first half is an enchanting depiction of the adventure of youth; the second half is full of tedious score-settling.)

It's eyebrow-raisingly frank. A few pages in, he tells us that when his aunt beat him as a boy it turned him on, and he has always yearned for such treatment in the bedroom. TMI, you might think (too much information, as we say in New York). But it is so engagingly written that you also want to say TMM (tell me more). Isn't frankness about sex ordinary nowadays – and doesn't today's writer go much further? Yes, but such frankness tends to be rather conformist – and compartmentalised. Rousseau still stands out for his determination to relate his love life (which is mostly romantic yearning) to his moral and psychological development, for his attempt at a holistic frankness.

What makes this book compelling is not just its frankness, but the spirit of defiance in which it is presented. Do you condemn me for these moral errors I have committed?, the reader is asked – sometimes explicitly. Do you claim to be my moral superior? If you condemn me, are you not condemning the honesty with which I present myself? "Let [readers] groan at my depravities and blush for my misdeeds," he says at the outset. "But let each one of them reveal his heart … with equal sincerity, and may any man, who dares, say: 'I was a better man than he'."

This remains a key motivation in autobiography: look at my faults, my embarrassing past errors, my lost years as a junkie or whatever, and agree that I am not defined by them, but rather by my good-heartedness, my striving to be myself. Agree, in fact, that my past faults are just proof of the fullness of my humanity. Judge me, says every memoirist – and affirm my humanity.

Nothing wrong with affirming each other's humanity, surely? Well, maybe secular confessionalism is subtly dishonest, in its habit of locating moral mistakes in the past and claiming to have learned from them. This deflects attention from the ongoing, ineradicable aspect of one's fallibility. Perhaps only a full-blooded, religious worldview can bear to keep that firmly in sight.

Having explored various issues in relation to Rousseau, no neat conclusion is possible. Except to say that he is perhaps the thinker who best illustrates the transition from a religious to a secular thought-world. He shows that this transition is a very incomplete one, for so much of secular humanism is constructed from religious material.