The cruel ferocity of barbarous zeal which, through so many centuries, wrought misery on mankind in the name of Christ, has been explained in many ways. Fanatics on the other side have denounced it as mere bloodthirstiness or selfish lust of power. Philosophers have traced it to the doctrine of exclusive salvation, through which it seemed the duty of those in authority to coerce the recalcitrant for their own benefit, and prevent them from leading other souls to perdition. Another school has taught that it arose from the survival of the atavistic notion of tribal solidarity, expanded into that of Christendom, making all share the guilt of sin offensive to God which they neglected to exterminate. Human impulses and motives, however, are too complex to be analyzed by a single solvent, even in the case of an individual, while here we have to deal with the whole Church, in its broadest acceptation, embracing the laity as well as the clergy. There is no doubt that the people were as eager as their pastors to send the heretic to the stake. There is no doubt that men of the kindliest tempers, the profoundest intelligence, the noblest aspirations, the purest zeal for righteousness, professing a religion founded on love and charity, were ruthless when heresy was concerned, and were ready to trample it out at the cost of any suffering. Dominic and Francis, Bonaventura and Thomas Aquinas, Innocent III. and St. Louis, were types, in their several ways, of which humanity, in any age, might well feel proud, and yet they were as unsparing of the heretic as Ezzelin da Romano was of his enemies. With such men it was not hope of gain or lust of blood or pride of opinion or wanton exercise of power, but sense of duty, and they but represented what was universal public opinion from the thirteenth to the seventeenth century.