16 Therefore came the Lord God to man as a sinner. You have heard the two names, both man and sinner. As man, he is of God; as a sinner, he is not of God. Let the moral evil in man be distinguished from his nature. Let that nature be owned, to the praise of the Creator; let the evil be acknowledged, that the physician may be called in to its cure. When the Lord then said, “He that is of God hears the words of God: ye therefore hear them not, because you are not of God.”
He did not distinguish the value of different natures, or find, beyond their own soul and body, any nature in men which had not been vitiated by sin; but foreknowing those who should yet believe, them He called of God, because yet to be born again of God by the adoption of regeneration. To these apply the words “He that is of God hears the words of God.” But that which follows, “You therefore hear them not, because you are not of God,” was said to those who were not only corrupted by sin (for this evil was common to all), but also foreknown as those who would not believe with the faith that alone could deliver them from the bondage of sin.
On this account He foreknew that those to whom He so spoke would continue in that which they derived from the devil, that is, in their sins, and would die in the impiety in which they resembled him; and would not come to the regeneration wherein they would be the children of God, that is, be born of the God by whom they were created as men. In accordance with this predestinating purpose did the Lord speak; and not that He had found any man among them who either by regeneration was already of God, or by nature was no longer of God.
Source: Tractates on the Gospel of John (New Advent)