During Holy Thursday Mass, Pope Francis washed the feet of migrants, three of whom were Muslims. Most Catholics understood this as a gesture of humility and brotherhood. That is how the Catholic press reported it—and that, undoubtedly, was the Pope's intention.
Many Muslims, however, may see it differently—not as a gesture of brotherhood, but as one of submission and surrender. The word "Islam" means "submission," and submission is what Islam expects of other faiths. Muslims consider Islam to be the supreme religion. To the extent that it tolerates the "People of the Book" (Christians and Jews), Islam tolerates them on the condition that they acknowledge its supremacy.
Historically, the People of the Book were expected to assume the status of dhimmis—second-class citizens with limited rights. The origin of this attitude can be found in several verses in the Koran, particularly 9:29, which says that the "People of the Book" are to be fought "until they pay the jizya with willing submission, and feel themselves subdued."