As an Oxford undergraduate in the early 19th century, Percy Bysshe Shelley developed an argument for the non-existence of God. He entitled it The Necessity of Atheism, and 2011 is the bicentenary of his being expelled from the university for printing it.

The argument itself is simple. If you have seen or heard God, then you must believe in God. If you haven't, then the only possible reasons to believe in God are reasonable argument or the testimony of others. The main argument given for believing in a deity – that the universe must have had a first cause – is not persuasive because there is no reason to believe either that the universe must have had a first cause or that this cause, if it existed, was a deity. The testimony of others – a third-rate source of knowledge in any case – is invariably contrary to reason. This is not least because it reports God as commanding belief, which would be irrational of God, given that belief is involuntary and not an act of will. So there is no reason to believe in God.

It is not a particularly shocking argument these days, but remembering this Shelley anniversary is important for other reasons.

Atheists today are too often castigated as materialistic calculators whose lack of spirituality sucks their universe empty of all beauty. Remembering Shelley's atheism gives us an opportunity to counter this stereotype and to reflect on the aesthetic of enchantment with which a non-theistic world-view can be associated. The works of Shelley join the novels, poems, songs, sculptures, paintings, architecture and plays of generations of godless artists in exposing the straw man of the desiccated rationalist for what it is, and showcasing a humanist vision of life.

More timely is a remembrance of the social and political consequences of Shelley's argument. In The Necessity of Atheism he reminds us of the mistake that people make when they think that "belief is an act of volition, in consequence of which it may be regulated by the mind" and the way that "continuing this mistake they have attached a degree of criminality to disbelief of which in its nature it is incapable". We cannot pillory someone for their disbelief – it is not an area in which choice operates.

Today in Britain, non-religious people are not thrown out of universities because they don't believe in God, but in other parts of the world many suffer this fate – and worse. There are still places where it is illegal to declare yourself as non-religious on your identity papers or official records.

One of the most upsetting stories I was ever told was by a young humanist from Saudi Arabia who grew up so frightened of what would happen if he spoke out loud about his beliefs to another person that the only outlet for his thoughts was to go on long walks away from all people, and speak his mind only to the air. In fact, he never spoke to another human being about his most fundamental beliefs until coming to Britain in his late 20s, and experiencing then for the first time what those of us who live in freedom take for granted: the joyful dynamic of testing and developing our own ideas in conversation and dialogue with others.

In this country the blasphemy laws have been abolished, but elsewhere our fellow men and women face death for speaking and thinking freely. Remembering Shelley – so eloquent himself on the subject of human solidarity – provides a dynamic call for us to address these injustices internationally.