In 988, Vladimir the Great, the Grand Prince of Kyiv (Kiev), embraced Christianity for himself and for the realm he had amassed. His domain of Kyivan Rus’ encompassed lands traversing much of modern-day Belarus, Ukraine and European Russia. One thousand years after the end of his reign, a latter-day Vladimir—a Putin rather than a prince—praised him as the “gatherer and protector of the Russian lands and a prescient statesman who laid the foundations of a strong, united, centralized state, resulting in the union of one great family of equal peoples, languages, cultures, and religions.”

Those...