Summa Theologiae IIaIIae, 2, 5

Whether man is bound to believe anything explicitly?

Objection 1. It would seem that man is not bound to believe anything explicitly. For no man is bound to do what is not in his power. Now it is not in man’s power to believe a thing explicitly, for it is written (Romans 10:14-15): “How shall they believe Him, of whom they have not heard? And how shall they hear without a preacher? And how shall they preach unless they be sent?” Therefore man is not bound to believe anything explicitly.

Reply to Objection 1. If we understand those things alone to be in a man’s power, which we can do without the help of grace, then we are bound to do many things which we cannot do without the aid of healing grace, such as to love God and our neighbor, and likewise to believe the articles of faith. But with the help of grace we can do this, for this help “to whomsoever it is given from above it is mercifully given; and from whom it is withheld it is justly withheld, as a punishment of a previous, or at least of original, sin,” as Augustine states (De Corr. et Grat. v, vi [Cf. Ep. cxc; De Praed. Sanct. viii.]).