(35) Take heed therefore that the light . . .—Better, See to it whether the light that is in thee be darkness. This takes the place in St. Luke’s report of St. Matthew’s (Luke 6:23) “If the light that is in thee be darkness, how great is that darkness!” The warning is one which calls men to self-scrutiny. They need to examine their primary beliefs, their very intuitions of right and wrong, lest all they do should be vitiated at its very source. The call to do this implies that they must have a Light by which to judge their light, a Standard by which to test their standard, and that Light and Standard are found in the teaching of the Light that lighteth every man, in the recorded words and acts of the Son of Man.

11:29-36 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.These verses are found in Matthew, but in a different connection. See the notes at Matthew 5:15; Matthew 6:22-23. 33-36. (See on [1640]Mt 5:14-16; [1641]Mt 6:22, 23.) But Lu 11:36 here is peculiarly vivid, expressing what pure, beautiful, broad perceptions the clarity of the inward eye imparts. See Poole on "Luke 11:34"

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.

Take heed therefore that the light which is in thee be not darkness.