The fact that a Sebastianist is someone who believes in something utterly unbelievable popped up on HH early on Tuesday morning:
...and there’s too good a story behind this one to leave it unexplained.
The Sebastian at the root of the word Sebastianist is Dom Sebastião de Portugal, or King Sebastian I, who ascended to the Portuguese throne in June 1557, at the age of just 3. Sebastian’s reign, despite his youth, was for the most part successful, and saw Portugal flourish both domestically and internationally with the success of its colonies in Africa and India.
But in 1578, the now 24-year-old King Sebastian embarked on a risky crusade against Morocco. Ignoring the best counsel of his advisors, Sebastian headed deep into the Moroccan desert to wage war, and on 4 August is presumed to have either been killed in battle at Alcácer Quibir, or else captured and executed soon afterward.
When news of the young king’s untimely death broke back in Portugal, many of the Portuguese people found it difficult to accept. As a result, rumours quickly began to emerge that Sebastian had in fact fled the battlefield in Morocco, or else had escaped captivity, and would one day return home to rule Portugal once more.
As the decades passed, the legend of the lost king endured. Imposters and pretenders to the throne claiming to be Sebastian came and went. Belief that the king was still alive, and would indeed return to save Portugal, persisted—to the extent that when John IV was crowned king in 1640, he had to agree to surrender the throne should Sebastian ever return (despite the fact that, by then, Sebastian would have been well in his 80s).
By the eighteenth century, more than a hundred years after his death, Sebastianism had become a pseudo-mystical cult, whose members believed the legendary Messiah-like king would rise from the dead to return Portugal to its former glory. Despite the cult’s increasingly impossible expectations, its numbers nevertheless continued to grow—notably after a renowned Sebastianist supporter supposedly predicted a powerful earthquake that struck Lisbon in 1755.
Today, however, the cult of Sebastianism has understandably largely been abandoned. But the term Sebastianist nevertheless remains in place in the language in a broader, allusive sense referring to someone who steadfastly believes something that is inherently unbelievable or impossible.