Roger Davidson's illustration of Tom Paine"Tis the business of little minds to shrink", wrote Thomas Paine in the famous first issue of his pamphlet sequence The American Crisis, "but he whose heart is firm, and whose conscience approves his conduct, will pursue his principles unto death. My own line of reasoning is to myself as straight and clear as a ray of light." True to his word, the great man died peacefully two hundred years ago this June in Greenwich Village, New York, aged 72, having led a life that saw him teeter often on the verge of financial, political and physical ruination for the sake of dedicating his prodigious abilities to the ideals of democracy, equality, republicanism and free thought.

A self-educated drifter from rural England, Paine had achieved little of note when he set sail, aged 37, for a new start in America. By his mid-forties, however, he could boast of having done what few can. At an age when most men tend to settle into stability and comfort, Paine stood up, struck out and changed the world. In the course of doing so, he became many things: poet, debater, proto-trade unionist, anti-slavery campaigner, revolutionary soldier, journalist, preacher, politician, economist, philosopher, prisoner, Biblical scholar, co-founder of the National Bank of America, Secretary for Foreign Affairs to the American Congress, representative for Calais in the French National Convention and, as he is best remembered, the inventor of modern democratic prose.

Paine glided into each of these roles with a cavalier ease, seemingly only questioning his purposes and abilities in his darkest moments - and even then only fleetingly. If the American Revolution had been smouldering since 1763, when King George, seeking to restrict the expansionist ambitions of the Thirteen Colonies, signed the Royal Proclamation, Paine, arriving from England in the winter of 1774, hurled a fistful of incendiary upon it of a strength and volatility that no one else had been able to muster. With the mass distribution of his legendary pamphlet Common Sense, the world was reborn. "I have the honest pride of thinking and ranking myself among the founders of a new Independent World," he told the American Minister of Finance, Robert Morris, in a moment of classic Paine immodesty.

The world had, of course, been waiting to be reborn. But Paine, possessed by his conviction that his was the voice not only of independence and national confidence but of universal freedom, spurred America to decide the moment of its own birth, to declare itself to be not only a nation but, in Paine's words, the "cause of all mankind". Amidst an atmosphere of uncertainty, terror and physical suffering, Paine took the morale of America in his hands, and made a profound contribution to its moral formation that is often overlooked today, just as it was all but dismissed by the fledgling American government in his own lifetime.

Neglect at the hands of the nation for whom he had very nearly sacrificed everything threw him into a brief depression in the late 1770s, but his self-confidence never faltered for long. Within a few years of the establishment of independence, Paine, who first cut the mould of the international revolutionary in which figures such as Leon Trotsky and Che Guevara would fashion themselves, had already begun to look to another continent for a new cause upon which to exercise his brilliance, admitting to his friend Henry Laurens that "it was neither the place nor the people [of America] but the Cause itself that irresistibly engaged me in its support."

Arriving in France in April 1791, where he anticipated the American Revolution was about to repeat itself (and that his role in it would be a suitably plum, historic one), Paine displayed his old self-certitude again, in full force and visible to all. To his supreme gratification, Rights of Man was creating a sensation in England and France, selling 50,000 copies in its first month, and going through various editions, some of them distributed by Paine himself at his own expense. The American ambassador to France, Gouverneur Morris, described him as "inflated to the eyes and big with a litter of revolutions". One of his hosts, Etiènne Dumont, spoke of his "egregious conceit and presumptuous self-sufficiency... He was drunk with vanity. If you believed him, it was he who had done everything in America... He told us roundly that, if it were in his power to annihilate every library in existence, he would do so without hesitation in order to eradicate the errors they contained, and commence with Rights of Man a new era."

Indeed, no subject was too great for Paine's intellectual and moral certitude, and in his later years he turned his mind towards yet another conflict of universal import, that between religion and common sense, performing with his Age of Reason (1794) a critique of the Christian faith that still startles today by virtue of its clarity, rationality and human and scientific emphasis, as well as its unabashed pithiness.

If many have seen his pugnacious personality and self-aggrandising tendency as disturbing anomalies in a great man, it's worth asking whether Paine could have been what he was without the attitude to himself and his place in the world that occasionally led him into self-absorption and folly. Like many great men, he didn't just tread a fine line between bluster and brilliance - there was no line there at all. Thomas Paine stands forth two hundred years after his death as an icon of freedom precisely because his prose exudes a confidence and somewhat brash gamesomeness that inspire the common man to value his natural common sense as equal to - or indeed greater than - that of the self-appointed gatekeepers of power and truth, whose infallibility, Paine insisted, by necessity blinds them to the common good.

As such, Paine, with the spirit of self-certitude that permeated his thought and his actions, is an immensely inspiring figure for today's secular rationalist. All of his writings constitute the most effective kind of critique of the spirit of authority and assertion that constitutes organised religion and has driven the bitter attacks launched upon free thought that have characterised the religious backlash against the New Atheists in recent years.

Among the charges pressed against this group - Christopher Hitchens, Richard Dawkins, Daniel Dennett, Sam Harris - is that of a combination of ignorance and arrogance, of much the same tenor as those levelled against Paine in his day. The New Atheists, it is argued by sophisticates like Terry Eagleton, John Gray and Madeleine Bunting, are vulgar and uncouth; they aren't well-versed enough in theological traditions to make true judgments on the nature of faith and the question of God's existence; they take, it is alleged, the claims of the faithful too literally, while remaining too conceited, cocksure and self-absorbed to grasp what is really meant by "God".

Paine's approach, of course, was not that of the atheist, but that of the deist who finds his creator-god embarrassed by that "history of the grossest vices, and . . . collection of the most paltry and contemptible tales" known as the Bible. He hoped for "happiness beyond this life", but wished to keep his faith untainted by prescriptive systems that claim to know infinitely more than they ever could, while insisting on the literal truth of those claims and adherence to the immolating doctrines that arise from them.

Paine demonstrated that to take the claims of religion in the spirit in which they were made, and not to be cowed by theological mystification, was every man's right, if not his obligation. This approach remains vital today because at the very root of the dispute between the faithful and the New Atheists is the fact that no matter what one's argument for faith, behind all convoluted theological "reasoning" a literal statement is always asserted - that a God exists whose purposes bear consequences for us, and that we should behave accordingly.

What the backlashers are essentially saying is that the New Atheists cannot comment on questions of faith because they are not initiated into the arcane "study or science which treats of God, His nature and attributes, and His relations with man and the universe" (the OED definition). Boiled down slightly, we are left with the sinister claim of "We know and you do not", an attitude that outraged Paine's powerful democratic instincts.

Let us atheists and humanists, then, take a leaf from Paine's great book, and stand firm in the confidence that the burden of proof lies with those who would make the most fantastic claims to knowledge of things unseen. This 8 June, raise a glass to old Tom, a steadfast ego in a world cowed by the pomp and pretension of the faithful and the powerful, and be grateful for his intellectual descendants and the perfectly justified self-certitude they bring to the most crucial debate of our times.