This essay deals with an essential issue in the emergence of modern consciousness: the litigious question between freedom and grace, in "The controversy between Erasmus and Luther about free will", -a little studied subject despite its scope in the history of religious and moral ideas. It analyzes in detail the tension between modern humanism and the reformed religion, and develops it in the various levels involved in it: in metaphysics, the conflict between predestination and freedom; in theology, the relationship between reason and faith, in anthropology, the opposition between the flesh and the spirit, and morally, the problem of evil and the liberation of servitude. And, moreover, it maintains in conclusion, that the correct understanding of this controversy and its overcoming constitutes the core of Christian humanism, as it was coined in the Renaissance.