Bede the Venerable — as excerpted in the Catena Aurea on Mark 9:14-29

“(ubi sup.) For no man at once reaches to the highest point, but in holy living a man begins with the least things that he may reach the great; for the beginning of virtue is different, from the progress and the perfection of it. Because then faith mounts up through the secret inspiration of grace, by the steps of its own meritsh, he who had not yet believed perfectly was at once a believer and an unbeliever.”