John 1:9

9.The true light was. The Evangelist did not intend to contrast the true light with the false, but to distinguish Christ from all others, that none might imagine that what is called light belongs to him in common with angels or men. The distinction is, that whatever is luminous in heaven and in earth borrows its splendor from some other object; but Christ is the light, shining from itself and by itself, and enlightening the whole world by its radiance; so that no other source or cause of splendor is anywhere to be found. He gave the name of the true light, therefore, to that which has by nature the power of giving light

Which enlighteneth every man. The Evangelist insists chiefly on this point, in order to show, from the effect which every one of us perceives in him, that Christ is the light. He might have reasoned more ingeniously, that Christ, as the eternal light, has a splendor which is natural, and not brought from any other quarter; but instead of doing so, he sends us back to the experience which we all possess. For as Christ makes us all partakers of his brightness, it must be acknowledged that to him alone belongs strictly this honor of being called light

This passage is commonly explained in two ways. Some restrict the phrase, every man, to those who, having been renewed by the Spirit of God, become partakers of the life-giving light. Augustine employs the comparison of a schoolmaster who, if he happen to be the only person who has a school in the town, will be called the teacher of all, though there be many persons that do not go to his school. They therefore understand the phrase in a comparative sense, that all are enlightened by Christ, because no man can boast of having obtained the light of life in any other way than by his grace. But since the Evangelist employs the general phrase, every man that cometh into the world, I am more inclined to adopt the other meaning, which is, that from this light the rays are diffused over all mankind, as I have already said. For we know that men have this peculiar excellence which raises them above other animals, that they are endued with reason and intelligence, and that they carry the distinction between right and wrong engraven on their conscience. There is no man, therefore, whom some perception of the eternal light does not reach.

But as there are fanatics who rashly strain and torture this passage, so as to infer from it that the grace of illumination is equally offered to all, let us remember that the only subject here treated is the common light of nature, which is far inferior to faith; for never will any man, by all the acuteness and sagacity of his own mind, penetrate into the kingdom of God. It is the Spirit of God alone who opens the gate of heaven to the elect. Next, let us remember that the light of reason which God implanted in men has been so obscured by sin, that amidst the thick darkness, and shocking ignorance, and gulf of errors, there are hardly a few shining sparks that are not utterly extinguished.