| Gill's Exposition of the Entire Bible Take heed therefore,.... By attending to the light of the Gospel, shining in the ministration of it, and do not neglect and despise it: that the light which is in thee be not darkness; lest being given up to a judicial blindness and hardness of heart, not only the light of nature, which the Jews had in common with the Gentiles, but even that notional light and knowledge of divine things, which they had by being favoured with an external revelation, the writings of the Old Testament, should be lost.
Vincent's Word StudiesThe light that is in thee Lit., the light, that, namely, which is in thee; thus emphasizing the inward light. See on Matthew 6:23.
Geneva Study BibleTake heed therefore that the light which is in thee be not darkness.
Matthew Henry's Concise Commentary11:29-36 Christ promised that there should be one sign more given, even the sign of Jonah the prophet; which in Matthew is explained, as meaning the resurrection of Christ; and he warned them to improve this sign. But though Christ himself were the constant preacher in any congregation, and worked miracles daily among them, yet unless his grace humbled their hearts, they would not profit by his word. Let us not desire more evidence and fuller teaching than the Lord is pleased to afford us. We should pray without ceasing that our hearts and understandings may be opened, that we may profit by the light we enjoy. And especially take heed that the light which is in us be not darkness; for if our leading principles be wrong, our judgment and practice must become more so. | 
