John Chrysostom

If then we have corrupted the understanding, which is able to let loose the passions, we have done violence to the whole soul, and suffer dreadful darkness, being blinded by the perversion of our understanding. Therefore adds he, Take heed, therefore, that the light which is in thee be not darkness. He speaks of a darkness which may be perceived, but which has its origin within itself, and which we every where carry about with us, the eye of the soul being put out. …

Catena Aurea by Aquinas ↗

Bede

Therefore, see to it that the light in you is not darkness. This means that the very intention of the heart, which is the light of the soul, should not be darkened by the fog of sins, but should be carefully considered with diligent discernment. As it is commanded elsewhere: With all vigilance, guard your heart, for from it flows the springs of life (Prov. IV).

On the Gospel of Luke ↗

Bonaventure

Third, there is added a commendable instruction, when he says: See therefore that the light which is in you be not darkness, that is, lest the intention by which you ought to be light be obscured by the darkness of vices. He speaks, moreover, to those converted to himself, of whom the Apostle says in Ephesians 5: "You were once darkness, but now light in the Lord: walk as children of light," that you may be children of that light "which enlightens every man coming into this world," John 1: not children of man, of whom John 3 says: …

Commentary on Luke, Chapter 11 ↗