John 1:3
But when you hear that “In Him was Life,” do not imagine Him a compound Being, since farther on he says of the Father also, “As the Father has Life in Himself, so has He given to the Son also to have Life”; now as you would not on account of this expression say that the Father is compounded, so neither can you say so of the Son. Thus in another place he says, that “God is Light”, and elsewhere (it is said), that He “dwells in light unapproachable”; yet these expressions are used not that we may suppose a compounded nature, but that little by little we may be led up to the highest doctrines. For since one of the multitude could not easily have understood how His life was Life Impersonate, he first used that humbler expression, and afterwards leads them (thus) trained to the higher doctrine. For He who had said that “He has given Him (the Son) to have life”; the Same says in another place, “I am the Life”; and in another, “I am the Light.” And what, tell me, is the nature of this “light”? This kind (of light) is the object not of the senses, but of the intellect, enlightening the soul herself. And since Christ should hereafter say, that “None can come unto Me except the Father draw him”; the Apostle has in this place anticipated an objection, and declared that it is He (the Son) who “gives light”; that although you hear a saying like this concerning the Father, you may not say that it belongs to the Father only, but also to the Son. For, “All things,” He says, “which the Father has are Mine.”
First then, the Evangelist has instructed us respecting the creation, after that he tells us of the goods relating to the soul which He supplied to us by His coming; and these he has darkly described in one sentence, when he says, “And the Life was the Light of men.” He does not say, “was the light of the Jews,” but universally “of men”: nor did the Jews only, but the Greeks also, come to this knowledge, and this light was a common proffer made to all. “Why did he not add 'Angels,' but said, 'of men?'?” Because at present his discourse is of the nature of men, and to them he came bearing glad tidings of good things.
“And the light shines in darkness.” He calls death and error, “darkness.” For the light which is the object of our senses does not shine in darkness, but apart from it; but the preaching of Christ has shone forth in the midst of prevailing error, and made it to disappear. And He by enduring death has so overcome death, that He has recovered those already held by it. Since then neither death overcame it, nor error, since it is bright everywhere, and shines by its proper strength, therefore he says,
“And the darkness comprehended it not.” For it cannot be overcome, and will not dwell in souls which wish not to be enlightened.
4. But let it not trouble you that It took not all, for not by necessity and force, but by will and consent does God bring us to Himself. Therefore do not thou shut your doors against this light, and you shall enjoy great happiness. But this light comes by faith, and when it has come, it lights abundantly him that has received it; and if you display a pure life (meet) for it, remains indwelling within continually. “For,” He says, “He that loves Me, will keep My commandments; and I and My Father will come unto him, and make Our abode with him.” As then one cannot rightly enjoy the sunlight, unless he opens his eyes; so neither can one largely share this splendor, unless he have expanded the eye of the soul, and rendered it in every way keen of sight.
But how is this effected? Then when we have cleansed the soul from all the passions. For sin is darkness, and a deep darkness; as is clear, because men do it unconsciously and secretly. For, “every one that does evil hates the light, neither comes to the light.” And, “It is a shame even to speak of those things which are done of them in secret.” For, as in darkness a man knows neither friend nor foe, but cannot perceive any of the properties of objects; so too is it in sin. For he who desires to get more gain, makes no difference between friend and enemy; and the envious regards with hostile eyes the man with whom he is very intimate; and the plotter is at mortal quarrel with all alike. In short, as to distinguishing the nature of objects, he who commits sin is no better than men who are drunk or mad. And as in the night, wood, lead, iron, silver, gold, precious stones, seem to us all alike on account of the absence of the light which shows their distinctions; so he who leads an impure life knows neither the excellence of temperance nor the beauty of philosophy. For in darkness, as I said before, even precious stones if they be displayed do not show their luster, not by reason of their own nature, but because of the want of discernment in the beholders. Nor is this the only evil which happens to us who are in sin, but this also, that we live in constant fear: and as men walking in a moonless night tremble, though none be by to frighten them; so those who work iniquity cannot have confidence, though there be none to accuse them; but they are afraid of everything, and are suspicious, being pricked by their conscience: all to them is full of fear and distress, they look about them at everything, are terrified at everything. Let us then flee a life so painful, especially since after this painfulness shall follow death; a deathless death, for of the punishment in that place there will be no end; and in this life they (who sin) are no better than madmen, in that they are dreaming of things that have no existence. They think they are rich when they are not rich, that they enjoy when they are not enjoying, nor do they properly perceive the cheat until they are freed from the madness and have shaken off the sleep. Wherefore Paul exhorts all to be sober, and to watch; and Christ also commands the same. For he who is sober and awake, although he be captured by sin, quickly beats it off; while he who sleeps and is beside himself, perceives not how he is held prisoner of it.
Source: Homilies on the Gospel of John (New Advent)