Ambrose of Milan
Catena Aurea · Lukasevangelium 17
Lk 17, Verses 5–6English original
And the apostles said unto the Lord, Increase our faith. And the Lord said, If ye had faith as a grain of mustard seed, ye might say unto this sycamine tree, Be thou plucked up by the root, and be thou planted in the sea; and it should obey you.
Or this is said because faith keeps out the unclean spirit, especially since the nature of the tree falls in with this meaning. For the fruit of the mulberry is at first white in the blossom, and being formed from thence grows red, and blackens as it gets ripe. The devil also having by transgression fallen from thewhite flower of the angelic nature and the bright beams of his power, grows terrible in the black odour of sin.