6 What we have just heard, appears now distinctly and clearly. “He that comes from above, is above all.” See what he says of Christ. What of himself? “He that is of the earth, is of earth, and speaks of the earth. He that comes from above is above all”— this is Christ; and “he that is of the earth, is of earth, and speaks of the earth”— this is John. And is this the whole: John is of the earth, and speaks of the earth? Is the whole testimony that he bears of Christ a speaking of the earth?
Are they not voices of God that are heard from John, when he bears witness of Christ? Then how does he speak of the earth? He said this of man. So far as relates to man in himself, he is of earth, and speaks of the earth; and when he speaks some divine things, he is enlightened by God. For, were he not enlightened, he would be earth speaking of earth. God's grace is apart by itself, the nature of man apart by itself. Do but examine the nature of man: man is born and grows, he learns the customs of men.
What does he know but earth, of earth? He speaks the things of men, knows the things of men, minds the things of men; carnal, he judges carnally, conjectures carnally: lo! It is man all over. Let the grace of God come, and enlighten his darkness, as it says, “You will lighten my candle, O Lord; my God, enlighten my darkness;” let it take the mind of man, and turn it to its own light; immediately he begins to say, as the apostle says, “Yet not I, but the grace of God that is with me;” and, “Now I live; yet not I, but Christ lives in me.” That is to say, “He must increase, but I must decrease.” Thus John: as regards John, he is of the earth, and speaks of the earth; whatever that is divine you have heard from John, is of Him that enlightens, not of him that receives.
Source: Tractates on the Gospel of John (New Advent)