The advent of the contemporary world has brought a series of new questions to moral theology, among where is the problem of defining what a mortal sin is. The many criticisms made by the revisionist theologians to the moral of the manuals, to the need to create an autonomous Christian morality, and to the conciliar solicitude of a reform in the studies of moral theology in the seminaries, were answered by the definition of mortal sin from the doctrine of the fundamental option. However, the promulgation of Veritatis Splendor stated that the instructions of the pastors of the Church were contrary to the thinking of theologians. In order to contribute to the solution of this difference between theology and pastoral, it s believed that the concept of mortal sin of Thomas Aquinas is the most adequate to dissolve the dispute. By the virtue of it s literary dependence on Aristotle s thought, it is intended to emphasize the patristic character of Aquinas s work so that the proposed solution will not be Hellenic, but essentially Christian.