On St. Patrick's Day, the Chicago River will turn an unnatural shade of green when city officials dump 40 pounds of dye into the water. Some 150,000 people will march up New York's Fifth Avenue to the strains of bagpipes, and from Montreal to Moscow revelers will deck themselves in green and swig pints of Guinness. As the saying goes, everyone's Irish on St. Patrick's Day—even if you're Korean, French or British.

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Most honorary Irish recognize St. Patrick as the bishop who introduced Christianity to the Emerald Isle. Legend tells us that he drove all of Ireland's snakes into the sea and used the native shamrock to teach new converts about the Christian trinity. But the historical Patrick "was nothing like the stories at all," says Philip Freeman, a historian at Iowa's Luther College and the author of St. Patrick of Ireland. "St. Patrick was not a miracle worker. He was just a regular guy who had a vision and worked very hard. But his real life is much more fascinating than the myths."

The little we know about St. Patrick comes from two long letters he wrote toward the end of his life, one assailing a British warlord who had kidnapped and murdered a group of Patrick's converts, and another defending himself against criticism from the British Church. The Patrick who wrote these letters is "a man full of insecurities and doubt," Freeman says, but "he's a man of tremendous faith at the same time."

In both letters, Patrick asks his readers to forgive his poor Latin, calling himself "the most unsophisticated and unworldly among all the faithful of God." The self-portrait that emerges from his writing has little in common with the St. Patrick most people wearing "Kiss me, I'm Irish" shirts imagine.

Though he would become the patron saint of Ireland, St. Patrick was actually British. A citizen of the Roman Empire, Patricius—his Latin name—was born into the nobility some time between A.D. 390 and 400. Though his family was Christian, they were not devout, and young Patrick was an atheist. He lived the comfortable life of a rich man's son until, at 15 years old, he was kidnapped by pirates, taken to Ireland and sold into slavery.

Patrick's fate was not unusual. Beginning in the 360s and continuing into the fifth century, Irish pirates raided villas up and down the west coast of Britain, capturing slaves to sell back in Ireland. "I was led away as a slave to Ireland, as were so many thousands of others," Patrick wrote later. "We had abandoned God...so God poured his anger on us and scattered us among the hordes of barbarians who live at the edge of the world."

The Romans considered Ireland to be the end of the Earth, a barely habitable island of ice and savagery. Some Greek and Roman geographers even alleged that the Irish were cannibals. Julius Caesar called the island "Hibernia," or "winter land," and anyone who found himself enslaved in this remote wasteland was considered as good as dead. Patrick spent the next six years of his life tending sheep for an Irish landowner, living in a hovel and enjoying no rights at all. To cope, he turned to prayer, earning the mock nickname "Holy Boy."

When he was 21, Patrick writes in one of his letters, he had a dream in which God told him to leave his master and escape Ireland. After walking some 185 miles through the Irish wilderness, fugitive Patrick obtained passage on a ship that eventually returned him to Britain and his family. There he had another dream, in which the Irish begged him to bring them Christianity. In Patrick's mind, he had no choice in the matter: He believed God was calling him. After training to become a priest, Patrick returned to the land of his captors.

A rural land governed by numerous tribal kings, 5th-century Ireland had little contact with the outside world. Except for some Christian slaves kidnapped from Britain (as Patrick had been), nearly everyone in Ireland practiced Celtic polytheism. Religious leaders called Druids acted as priests and magicians, and naturally they opposed foreign missionaries. Patrick's mission was a daunting one, but over the next few decades he preached the Gospel, working his way from farm to farm and village to village—guided, he believed, by prophetic dreams. In time, he established an active Church of Ireland.

Patrick probably died sometime in the 460s, and shortly thereafter he fell into obscurity. But as the Irish Church grew, vague memories of Patrick developed into fanciful stories. Christian writers asserted that he had fought Druid magic and performed various other feats, wowing the Irish with supernatural powers. During the Middle Ages, biographies of saints were intended to inspire faith rather than record historical events, and the real Patrick was soon lost in the legend. "Saints were always working miracles; that was standard fare," explains Philip Freeman. "The snakes, for example, are just a representation of evil. Patrick drove the old evil pagan ways out of Ireland and brought Christianity there."

St. Patrick's Day falls on March 17, which is traditionally thought to be the day of his death, although that is probably a guess. For centuries, St. Patrick's Day was simply a religious holiday, celebrated in church rather than at the pub; in fact, in Ireland pubs were closed on March 17 until the 1970s.

It was not until Irish immigrants began coming to America that St. Patrick's Day took off as the international, secular celebration we know today. Boston's Charitable Irish Society held the first St. Patrick's Day parade in 1737. Irish New Yorkers first marched in 1762, and Philadelphia caught on in 1780. For early Irish-Americans, the holiday was a way of looking back fondly on the old country and expressing ethnic pride, and over the years more and more people have joined in the fun. The Irish themselves recently adopted many American traditions surrounding St. Patrick's Day, largely as a ploy to attract tourists.

What would St. Patrick think of the way his holiday is celebrated today? "He would probably be astounded," says Freeman. "He was not the sort of man who liked to draw attention to himself, and he would not have been comfortable with all the parades and the drinking. I think he would be very surprised."