In his chapter, The Origin of Papacy, Guignebert outlines the historical rise of the pope in the Catholic church, providing discussion of inside and outside influences that defined and legitimized the pope's power. It didn't happen overnight, but over a few hundred years and through several generations of bishops of Rome, the office of pope evolved. Here's the passage in that chapter, which struck me, if not like a thunderbolt, at least with a sizzle as I recognized the repeat of history.
The second of these favoring outside influences was the Crusades inspired by him, which clearly set him from the beginning of the eleventh century at the head of all the Christians fighting the infidel. The Crusades did not succeed, but their early ephemeral triumph, and the years they lasted, and then too the hope, always springing up again after each setback, of a forthcoming new crusade, enabled the Pope to keep up indefinitely his attitude of supreme head of all believers, and the active champion of the faith. ...the Crusades enabled Western peoples to rediscover the East.