Father Pegues discusses St. Thomas Aquinas’ teaching on grace, as presented in the saint’s Treatise On Grace (Summa Theologica I-II, 109-114).
Father Pegues notes that laws help a person to live a virtuous life by avoiding a life of sin. But, law alone is not sufficient. The help of grace is also necessary. Grace is “a special help from God that assists him to do good and to avoid evil. Of himself, that is relying on the principle of his nature given to him by God, and upon the other natural helps around him, man can accomplish certain good acts and avoid certain evil acts even in the moral order or in the domain of virtue; but if God by His grace does not heal human nature which was wounded by sin, man would never be able to accomplish even in the order of natural virtue all the good required of him or to avoid all evil; moreover, in the order of supernatural virtue or as regards the good life that is to win heaven, man by his sole nature, without grace, can do absolutely nothing.”
The grace of the supernatural order implies two things: “a supernatural state of soul, and supernatural motions of the Holy Spirit.” The supernatural state of soul consists of “certain qualities introduced and preserved therein by God which in a sense make the very being and faculties of man divine.” The fundamental quality which divinizes his being is sanctifying grace, and the supernatural qualities which divinize the human faculties are the virtues and the gifts of the Holy Spirit.
The Holy Spirit moves a person “to prepare himself to receive grace if he has it not, or to make daily progress therein if he already possesses it.” This movement, which is a type of actual grace, “cannot produce its full effect in us, in spite of us, and without our response. . . . Our free will must co-operate with the motion of actual grace.” This co-operation is called “correspondence with grace.”
When, in a state of sanctifying (habitual) grace, one performs an action in cooperation with actual grace, that action is meritorious. Father Pegues mentions two kinds of merit: merit de condigno and merit de congruo. Merit de condigno is “merit that demands recompense by right and in strict justice.” To be meritorious de condigno, “the act must be done under the impulse of actual grace; it must proceed from sanctifying grace by the virtue of charity; and it must tend towards the acquisition of eternal life for itself alone, or further, towards the increase of grace and of the virtues.” Merit de congruo is what God, on account of His friendship with a person, “deems fitting and in accord with His wishes” to grant that person. We can merit for others de congruo, but not de condigno; Christ alone merited for others de condigno.
Quotations from Thomas Pegues, Catechism of the “Summa Theologica” of Saint Thomas Aquinas, trans. Aelred Whitacre (New York: Benziger, 1922).