“Tractatus Theologico-Politicus,” an anonymous Latin treatise that declared the best policy in religious matters to be “allowing every man to think what he likes, and say what he thinks.” In the preface, the author gave thanks for the “rare happiness of living in a republic, where everyone’s judgment is free and unshackled, where each may worship God as his conscience dictates, and where freedom is esteemed before all things dear and precious.” But the fact that the author withheld his name, and that the book’s Amsterdam publisher claimed on the title page that it had been printed in Hamburg, told another story. The author and the publisher were well aware that their unshackled judgment could put them in shackles.

These feints couldn’t stop readers, or the authorities, from quickly figuring out that the “Tractatus” was the work of Baruch Spinoza. Although Spinoza, then in his late thirties, had previously published only one book, a guide to the fashionable philosophy of René Descartes, he was one of Amsterdam’s most notorious freethinkers. As a young man, he had been expelled from the city’s Jewish community for his heretical views on God and the Bible. (He published under the name Benedictus de Spinoza, Benedictus being the Latin equivalent of Baruch, which means “blessed” in Hebrew.) Living a quiet, solitary existence, supporting himself by grinding lenses for microscopes and telescopes, Spinoza developed his ideas into a comprehensive philosophical system, which he shared with a circle of friends in letters and conversations. When Koerbagh was interrogated, he was asked whether he had fallen under Spinoza’s malign influence. He acknowledged that they were friends, but insisted that they had never discussed ideas—even though what he wrote about God closely resembled what Spinoza had been saying for years.

Ministers in several cities immediately forbade booksellers to carry the “Tractatus,” and, in 1674, it was officially banned in the Netherlands, along with Thomas Hobbes’s “Leviathan.” Under the circumstances, Spinoza’s praise of Dutch freedom might well sound sarcastic. But the truth is that, compared with most of Europe in the seventeenth century, the Netherlands really was a haven of tolerance. In Spain or Italy, a book like Spinoza’s could get its author burned by the Inquisition; as it was, the attacks were aimed at his ideas, not his life. His praise of his country is better seen as a kind of appeal: Perhaps no country in Europe was truly free, but the Netherlands might be if it tried.

For Ian Buruma, a writer and historian and a former editor of The New York Review of Books, it is Spinoza’s dedication to freedom of thought—what he called libertas philosophandi—that makes him a thinker for our moment. In his new book, “Spinoza: Freedom’s Messiah,” a short biography in Yale University Press’s Jewish Lives series, Buruma observes that “intellectual freedom has once again become an important issue, even in countries, such as the United States, that pride themselves on being uniquely free.”