Summa contra Gentiles3.151 Chapter CLI. That sanctifying grace causes in us the love of God

From what has been said it follows that by the assistance of sanctifying grace man is enabled to love God. For sanctifying grace is an effect in man of the divine love. Now, the proper effect of the divine love in man would seem to be that man loves God. Because the chief thing in the intention of one who loves, is that he be loved in return: since the endeavour of the lover tends especially to draw the beloved to love of him: and unless he accomplishes this, love must cease. Consequently the effect of sanctifying grace in man is that he loves God.

Again. Things that have a common end must be united in so far as they are directed to that end: hence in a state men are joined together in concord that they may ensure the good of the commonwealth; and soldiers, when engaged in battle, must needs be united together, and act in unison in order to achieve the victory which is their common end. Now, the last end to which man is conducted by the assistance of divine grace, is the vision of God in His essence, which, vision is proper to God himself: so that this final good is communicated to man by God. Consequently man cannot attain to this end unless he be united to God by conformity of will. And this is the proper effect of love; since it is proper to friends to like and dislike the same things, and to have common joys and griefs. Therefore by sanctifying grace man becomes a lover of God: because by it man is directed to an end communicated to him by God.

Also. Since the end and the good is the proper object of the appetite or affections, it follows that man’s affective faculty is perfected chiefly by sanctifying grace which directs man to his last end. Now, the chief perfection of the affective faculty is love. A sign of which is that every affective movement originates in love: for no one desires, or hopes, or rejoices, save on account of a good that he loves; and in like manner no one shuns, or fears, or grieves, or is angry, except on account of something contrary to that which he loves. Therefore the principal effect of sanctifying grace is that man loves God.

Further. The form by virtue of which a thing is directed to its end, likens that thing somewhat to that end: thus a body by virtue of the form of gravity assumes a certain igo likeness to and conformity with the place to which its movement tends naturally. Now, we have already shown that sanctifying grace is a form residing in man, and directing him to his last end, which is God. Therefore grace makes man like to God. But likeness is the cause of love; because like loves like. Therefore grace makes man a lover of God.

Moreover. Operation, to be perfect, must be constant and prompt. Now, this is the chief effect of love, which makes even hard things seem light. Since then sanctifying grace is needed for the perfection of human actions, as stated above, it follows of necessity that this same grace produces in us the love of God.

Hence the Apostle says (Rom. v. 5): The chanty of God is poured forth in our hearts, by the Holy Ghost who is given to us. Moreover our Lord promised the vision of Himself to those who love Him, saying (Jo. xiv. 21): He that loveth me, shall be loved of My Father: and I will love him, and will manifest myself to him.

It is evident, then, that grace which directs us to that end which is to see God, causes in us the love of God.