John 8:12 - Matthew Poole's English Annotations on the Holy Bible

I am the light of the world; this is what John the Baptist had said of Christ before, 1 Thessalonians 1:4,5, and what Christ saith of himself afterward, 1 Thessalonians 9:5. It was prophesied of him, that he should be a light to the Gentiles, and God's salvation to the ends of the earth, Isaiah 46:6. And old Simeon saith of him, Luke 2:32, that he was to be a light to lighten the Gentiles, and the glory of his people Israel. Light is a thing glorious in itself, and communicative of itself unto others to guide them. So as Christ is most aptly compared to light, and spoken of under that notion; as for his own innate glory, so for the communicativeness of himself to creatures; which latter appeareth to be chiefly here intended: for he saith, that he who followed him, believing his doctrine, and obeying his precepts, living according to his direction and his example, should not be at a loss how to guide himself, nor remain in the darkness of sin, ignorance, and spiritual death; but should have that light which bringeth life along with it, and is sufficient to guide a man in all the works of a spiritual life, and at last bring him to life eternal.