Some Muslims will never speak of "converts" but only "reverts" because they believe that everyone is born a Muslim, even if some babies have this truth hidden from them by their parents who tell them they're Christians or atheists. And there's a style of atheist rhetoric that makes exactly the same point. To take two random examples from my recent Twitter stream: Joan Smith wrote: "I'm not convinced there are Muslim or Christian children. They have religious parents, but should be able to decide when they grow up." And Richard Dawkins wrote: "When you say X is the fastest growing religion, all you mean is that X people have babies at the fastest rate. But babies have no religion."

But there are no atheist babies, and certainly no agnostic ones. This is for two reasons. The first is that if we're going to be consistent, and to demand that babies only be ascribed identities that they themselves embrace, there are no German, British or Chinese children either. There are simply the children of German and English and Chinese parents, who will in due course learn the habits and the rules of the cultures around them and grow into their parents' language, nationality, food habits – and religious opinions. The way in which they express these will become more subtle and more interesting as they grow up – or at least we can hope it will – but the fact remains that babies are entirely anchored in the world by their parents.

But you don't get Dawkins and Smith complaining because people talk about "Chinese babies". They think religion is different. Well, it is. For one thing, and despite the existence of loathsome and barbaric laws against apostasy, in most of the world it's much easier to change your religion than your language or nationality. It is generally accepted that changing your religion is a human right, but changing your nationality is not. The big difference is that religions usually make it hard to leave and nationalities usually make it hard to enter. But in neither case does an individual get to choose as if no one else were involved. To imply that babies have a default theological position of atheism is as silly as assuming that they have a default language or nationality.

Of course, in an environment where religion is regarded as weird and old-fashioned, children grow up atheist because that's what their parents are. They don't think about it. They may have profoundly superstitious and unscientific beliefs, but they will think of these as rational and atheist because that's what – they know – all decent people are.

This is a perfectly sensible piece of conformist time-saving – life's too short to live without prejudice – but it isn't a reasoned rejection of belief after serious consideration of its possible truth.

There is another reason why babies can't be atheists or agnostics. Everything we know from science shows that supernaturalism comes naturally to children. It is not just that they believe much of what their parents and the surrounding societies tell them: they show a preference for remembering and transmitting stories that defy scientific rationality. So do we all, unless we train ourselves out of it.

To reach the state where you can really reflect critically on your own beliefs – rather than simply understanding that your parents are deluded old fools – takes a long time if it ever happens at all. As Bertrand Russell observed, many people would rather die than think and most of them do. And that is why no one can really be called an atheist or an agnostic until they have grown up.