What Lights Means
But why, after saying, “which lights every man,” should he add, “that comes into the world,” — the clause which has suggested the opinion that He enlightens the minds of newly-born babes while the birth of their bodies from their mother's womb is still a recent thing? The words, no doubt, are so placed in the Greek, that they may be understood to express that the light itself “comes into the world.” If, nevertheless, the clause must be taken as expressing the man who comes into this world, I suppose that it is either a simple phrase, like many others one finds in the Scriptures, which may be removed without impairing the general sense; or else, if it is to be regarded as a distinctive addition, it was perhaps inserted in order to distinguish spiritual illumination from that bodily one which enlightens the eyes of the flesh either by means of the luminaries of the sky, or by the lights of ordinary fire.
So that he mentioned the inner man as coming into the world, because the outward man is of a corporeal nature, just as this world itself; as if he said, “Which lights every man that comes into the body,” in accordance with that which is written: “I obtained a good spirit, and I came in a body undefiled.” Or again, the passage, “Which lights every one that comes into the world,”— if it was added for the sake of expressing some distinction—might perhaps mean: Which lights every inner man, because the inner man, when he becomes truly wise, is enlightened only by Him who is the true Light.
Or, once more, if the intention was to designate reason herself, which causes the human soul to be called rational (and this reason, although as yet quiet and as it were asleep, for all that lies hidden in infants, innate and, so to speak, implanted), by the term illumination, as if it were the creation of an inner eye, then it cannot be denied that it is made when the soul is created; and there is no absurdity in supposing this to take place when the human being comes into the world.
But yet, although his eye is now created, he himself must needs remain in darkness, if he does not believe in Him who said: “I have come a Light into the world, that whosoever believes in me should not abide in darkness.” And that this takes place in the case of infants, through the sacrament of baptism, is not doubted by mother Church, which uses for them the heart and mouth of a mother, that they may be imbued with the sacred mysteries, seeing that they cannot as yet with their own heart “believe unto righteousness,” nor with their own mouth make “confession unto salvation.” There is not indeed a man among the faithful, who would hesitate to call such infants believers merely from the circumstance that such a designation is derived from the act of believing; for although incapable of such an act themselves, yet others are sponsors for them in the sacraments.
Source: Merits and Remission of Sin, and Infant Baptism (New Advent)