The head of Christians does not, as a rule, have power to punish secular wrongs with a capital penalty and other bodily penalties and it is for thus punishing such wrongs that temporal power and riches are chiefly necessary; such punishment is granted chiefly to the secular power. The pope therefore, can, as a rule, correct wrongdoers only with a spiritual penalty. It is not, therefore, necessary that he should excel in temporal power or abound in temporal riches, but it is enough that Christians should willingly obey him.

"A Letter to the Friars Minor" (1334) as translated in A Letter to the Friars Minor and other Writings (1995) edited by A. S. McGrade and John Kilcullen, p. 204.