4 Attend to what follows: “I have come a light into the world, that whosoever believes in me should not abide in darkness.” He said in a certain place to His disciples, “You are the light of the world. A city that is set on a hill cannot be hid. Neither do men light a candle, and put it under a bushel, but on a candlestick; that it may give light to all that are in the house: so let your light shine before men, that they may see your good works, and glorify your Father who is in heaven:” but He did not say to them, You have come a light into the world, that whosoever believes in you should not abide in darkness. Such a statement, I maintain, can nowhere be met with. All the saints, therefore, are lights, but they are illuminated by Him through faith; and every one that becomes separated from Him will be enveloped in darkness. But that Light, which enlightens them, cannot become separated from itself; for it is altogether beyond the reach of change. We believe, then, the light that has thus been lit, as the prophet or apostle: but we believe him for this end, that we may not believe in that which is itself enlightened, but, with him, on that Light which has given him light; so that we, too, may be enlightened, not by him, but, along with him, by the same Light as he. And when He says, “That whosoever believes in me may not abide in darkness,” He makes it sufficiently manifest that all have been found by Him in a state of darkness: but that they may not abide in the darkness wherein they have been found, they ought to believe in that Light which has come into the world, for thereby was the world created.
5. “And if any man,” He says, “hear my words, and keep them not, I judge him not.” Remember what I know you have heard in former lessons; and if any of you have forgotten, recall it: and those of you who were absent then, but are present now, hear how it is that the Son says, “I judge him not,” while in another place He says, “The Father judges no man, but has committed all judgment unto the Son;” namely, that thereby we are to understand, It is not now that I judge him. And why not now? Listen to the sequel: “For I am not come,” He says, “to judge the world, but to save the world;” that is, to bring the world into a state of salvation. Now, therefore, is the season of mercy, afterwards will be the time for judgment: for He says, “I will sing to You, O Lord, of mercy and judgment.”
Source: Tractates on the Gospel of John (New Advent)