It’s difficult to pinpoint a precise moment when the office of the pope began to lose its vast political power, which had long placed the Holy See above even the kings and emperors of Europe, but has since declined to the point that now-retiring Pope Benedict XVI found few political accomplishments in his reign. But one day that stands out is Dec. 2, 1804.

A few weeks earlier, French voters had overwhelmingly approved a referendum elevating Napoleon Bonaparte from first consul to emperor, the beginning of the end of France’s democratic revolution. His coronation was to proceed in the manner of all Catholic monarchs, who still ruled most of Europe: he would kneel before the pope, then Pius VII, to receive a crown and blessing. The symbolism of the coronation reflected centuries of European political tradition, in which the Catholic church formally conferred royalty with the divine blessing that was thought necessary to rule; the church, in its power, had at times competed openly with those same monarchs....