John Chrysostom — as excerpted in the Catena Aurea on Mark 4:21-25

“(non occ.) After the question of the disciples concerning the parable, and its explanation, He well subjoins, And he said unto them, Is a candle brought, &c. As if he said, A parable is given, not that it should remain obscure, and hidden as if under a bed or a bushel, but that it should be manifested to those who are worthy. The candle within us is that of our intellectual nature, and it shines either clearly or obscurely according to the proportion of our illumination. For if meditations which feed the light, and the recollection with which such a light is kindled, are neglected, it is presently extinguished.”