take heed, then, lest the light that is in thee be darkness;

Consider therefore whether the light that is in you is anything but mere darkness.

So take care that the light which is in you is not dark.

Therefore, take care, lest the light that is within you become darkness.

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.