Why not by reason? The answer is given in the next argument. The work of our salvation must be all of a piece. The essential means to a supernatural end must be themselves supernatural. Grace however is no miraculous illumination of the intellect, no private revelation of the mysteries of God: it supposes the Christian tradition handed down from man to man, rationally taught and rationally grasped: but it disposes us to accept that tradition as we hear it, and it assists the teaching Church to preserve it.
Of God and His Creatures: 3.153