This chapter explicates the relation between moral philosophy and moral theology in Reformed orthodoxy by analysis of the early and influential Protestant reformer Pietro Martire Vermigli (1499-1562). It attempts to make a contribution to the more basic issue of the relation between faith and reason in Reformed orthodoxy and to solve contradictory interpretations. For claims of ethical knowledge on the basis of human reason on the one hand and claims of ethical knowledge on the basis of divine revelation on the other, raise the issue of the relation between reason and faith. Moral discourse is moreover "discourse of happiness", because as completion of human nature happiness is the ultimate end and highest good of human action. Thus the human act is considered either under the form of human reason in moral philosophy or the form of divine reason in moral theology. Keywords: ethical knowledge; moral philosophy; moral theology; Pietro Martire Vermigli; reformed orthodoxy