Innocent III, the pope who moved to crush the Cathars (see last week's Christian History Corner), also accidentally struck at an even bigger "heresy": Eastern Orthodoxy. Though personally tolerant of Eastern Christian rites, he mustered the disastrous Fourth Crusade, which in 1204 forever impaired East-West relations by sacking Constantinople. However, as historian Tia M. Kolbaba ably demonstrates in The Byzantine Lists: Errors of the Latins (University of Illinois), the gulf between Western "Latins" and Eastern "Greeks" goes much deeper.

The Byzantine lists, which detailed the errors of the Western church, were written by educated Greek clergy for Greek lay persons to discourage them from picking up Latin practices from visiting crusaders or other contacts with the West. The first list (not counting a list-like 867 encyclical from Patriarch Photios) was written by Patriarch Keroularios, or Cerularius, in 1054, in reaction to a list of Greek errors handed to him by Pope Leo IX's representative, Cardinal Humbert. Keroularios and Humbert excommunicated each other, leading to the familiar designation of 1054 as the date of the Great Schism. Keroularios's list would be copied and expanded upon, then joined with other lists, through 1281. The number of Latin errors identified eventually reached 75.

According to Kolbaba, historians have never really studied the lists because of their unusual content: a mixture of theological, liturgical, and seemingly personal disagreements. Keroularios's list accuses Latins of, among other things, using unleavened bread in the Eucharist, eating unclean meats, shaving, adding "and the Son" to the Nicene Creed (the "filioque" clause), forbidding priests to marry, allowing bishops to wear rings, and ...