In the previous article, I considered Rousseau's political radicalism, at some speed. I want to offer a summarizing reflection on that theme, before moving on to his religious thought.

Political justice, said Rousseau, depends on an understanding that state power belongs to the people, exists to serve the common good. What is this vision? Where does it come from? It is motivated by a moral idealism rooted in Judeo-Christian tradition (social justice, concern for the poor, hostility to luxury, the equal worth of all human lives). But its practical side is derived mainly from Plato's Republic. It's a potent conjoining. Rousseau is perhaps the principal pioneer of the idea that a more moral politics must be established and sustained through force – as all political order is.

His political radicalism annoyed the authorities, but what really provoked them was his religious radicalism. In his novel Emile he put his thoughts into the mouth of an ultra-liberal priest, the original trendy vicar (in his youth Rousseau was deeply influenced by a real-life version of this figure). He explains that God is the creator of the orderly universe, that his rules are written in our hearts, in the form of conscience, that virtuous action brings true happiness. We are made for virtue – though we are free to misunderstand this and do evil. Because "the greatest ideas of the divinity come to us from reason alone", revealed religion is dubious, a source of conflict and error. It is a slur on God to associate him with anger and vengeance, and narrow intolerant doctrines. "If one had listened only to what God says to the heart of man, there would never have been more than one religion on earth."

But traditional religion cannot be simply rejected, says this fictional priest – otherwise he would have left his job. The truth of rational religion must be propagated through traditional religious forms – this is its necessary packaging (an idea later developed by Kant and Hegel). People need structures, traditions, ritual practices, through which to relate to divine morality.

On one level this is a conventional restatement of deism. But Rousseau's emphasis on the emotional appeal of this creed breathed new life into it. He rescued it from narrow rationalism, and associated it with a deepening of humanity. (By putting it in the mouth of a priest he presents it as primarily a spiritual phenomenon.) He thus prepared the way for the poetic deism of Wordsworth, for example.

Not many people nowadays identify with deism. In fact it is almost universally scorned: as a timid compromise, for those who can no longer believe in religion but are not quite ready to take leave of God. Atheists of course see themselves as going a daring step further. To Christians, it is a diluted form of Christianity emptied of crucial concepts such as revelation, grace and sin.

I suggest that both the atheist and the Christian should be a bit less scornful, and see how deeply they are indebted to this huge intellectual movement.

Atheism is less distinct from deism than it thinks. It inherits the semi-Christian assumptions of this creed.

Atheism derives from religion? Surely it just says that no gods exist, that rationalism, or 'scientific naturalism', is to be preferred to any form of supernaturalism. Actually, no: in reality what we call atheism is a form of secular humanism; it presupposes a moral vision, of progressive humanitarianism, of trust that universal moral values will triumph. (Of course there is also the atheism of Nietzsche, which rejects humanism, but this is not what is normally meant by 'atheism').

So what we know as atheism should really be understood as an offshoot of deism. For it sees rationalism as a benign force that can liberate our natural goodness. It has a vision of rationalism saving us, uniting us. For example, AC Grayling, in his recent book The God Argument: The Case Against Religion and for Humanism, argues that, with the withering of religion, 'an ethical outlook which can serve everyone everywhere, and can bring the world together into a single moral community, will at last be possible'. This is really Rousseau's idea, that if we all listened to our hearts, there would be 'one religion on earth'.

On one hand atheism is more coherent than deism – it neatly eliminates the supernatural. But on the other hand it has less self-knowledge: it does not understand that it remains fuelled by a religious-based vision of human flourishing.

Next time I will suggest that Rousseau's deism also has something to teach Christians who are over-confident in their brave distinctness from secular humanism.