8 “He that comes from heaven is above all; and what He has seen and heard, that testifies He; and His testimony no man receives.” If no man, to what purpose came He? He means, no man of a certain class. There are some people prepared for the wrath of God, to be damned with the devil; of these, none receives the testimony of Christ. For if none at all, not any man, received, what could these words mean, “But he that received His testimony has set to his seal that God is true”? Not certainly, then, no man, if you say yourself, “He that received His testimony has set to his seal that God is true.” Perhaps John, on being questioned, would answer and say, I know what I have said, in saying no man. There are, in fact, people born to God's wrath, and thereunto foreknown. For God knows who they are that will and that will not believe; He knows who they are that shall persevere in that in which they have believed, and who that shall fall away; and all that shall be for eternal life are numbered by God; and He knows already the people set apart. And if He knows this, and has given to the prophets by His Spirit to know it, He gave this also to John. Now John was observing, not with his eye—for as regards himself he is earth, and speaks of earth—but with that grace of the Spirit which he received of God, he saw a certain people, ungodly, unbelieving. Contemplating that people in its unbelief, he says, “His testimony, who came from heaven, no man receives.” No man of whom? Of them who shall be on the left hand, of them to whom it shall be said, “Go into the everlasting fire, which is prepared for the devil and his angels.” Who are they that do receive it? They who shall be at the right hand, they to whom it shall be said, “Come, you blessed of my Father, receive the kingdom which is prepared for you from the beginning of the world.” He observes, then, in the Spirit a dividing, but in the human race a mingling together; and that which is not yet separated locally, he separated in the understanding, in the view of the heart; and he saw two peoples, one of believers, one of unbelievers. Fixing his thought on the unbelievers, he says, “He that comes from heaven is above all; and what He has seen and heard, that He testifies and no man receives His testimony.” He then turned his thought from the left hand, and looked at the right, and proceeded to say, “He that received His testimony has set to his seal that God is true.” What means “has set to his seal that God is true,” if it be not that man is a liar, and God is true? For no human being can speak any truth, unless he be enlightened by Him who cannot lie. God, then, is true; but Christ is God. Would you prove this? Receive His testimony and you find it. For “he that has received His testimony has set to his seal that God is true.” Who is true? The same who came from heaven, and is above all, is God, and true. But if you do not yet understand Him to be God, you have not yet received His testimony: receive it, and you put your seal to it; confidently you understand, definitely you acknowledge, that God is true.
9. “For He whom God has sent speaks the words of God.” Himself is the true God, and God sent Him: God sent God. Join both, one God, true God sent by God. Ask concerning them singly, He is God; ask concerning them both, they are God. Not individually God, and both Gods; but each individual God, and both God. For so great is the charity of the Holy Spirit there, so great the peace of unity, that when you question about them individually, the answer to you is, God; when you ask concerning the Trinity, you get for answer, God. For if the spirit of man, when it cleaves to God, is one spirit, as the apostle openly declares, “He that is joined to the Lord is one spirit;” how much more is the equal Son, joined to the Father, together with Him one God! Hear another testimony. You know how many believed, when they sold all they had and laid it at the apostles' feet, that it might be distributed to each according to his need; and what says the Scripture of that gathering of the saints? “They had one soul and one heart in the Lord.” If charity made one soul of so many souls, and one heart of so many hearts, how great must be the charity between the Father and the Son! Surely it must be greater than that between those men who had one heart. If, then, the heart of many brethren was one by charity, if the soul of many brethren was one by charity, would you say that God the Father and God the Son are two? If they are two Gods, there is not the highest charity between them. For if charity is here so great as to make your soul and your friend's soul one soul, how can it be then that the Father and the Son is not one God? Far be unfeigned faith from this thought. In short, how excellent that charity is, understand hence: the souls of many men are many, and if they love one another, it is one soul; still, in the case of men, they may be called many souls, because the union is not so strong. But there it is right for you to say one God; two or three Gods it is not right for you to say. From this, the supreme and surpassing excellency of charity is shown you to be such, that a greater cannot be.
10. “For He whom God has sent speaks the words of God.” This, of course, he said of Christ, to distinguish himself from Christ. What then? Did not God send John himself? Did he not say himself, “I am sent before Him”? And, “He that sent me to baptize with water”? And is it not of John that it is said, “Behold, I send my messenger before You, and he shall prepare Your way”? Does he not himself speak the words of God, he of whom it is said that he is more than a prophet? Then, if God sent him too, and he speaks the words of God, how do we understand him to have distinctly said of Christ, “He whom God has sent speaks the words of God”? But see what he adds: “For God gives not the Spirit by measure.” What is this, “For God gives not the Spirit by measure”? We find that God does give the Spirit by measure. Hear the apostle when he says, “According to the measure of the gift of Christ.” To men He gives by measure, to the only Son He gives not by measure. How does He give to men by measure? “To one is given by the Spirit the word of wisdom; to another the word of wisdom according to the same Spirit; to another faith by the same Spirit; to another prophecy; to another discerning of spirits; to another kinds of tongues; to another the gift of healing. Are all apostles? Are all prophets? Are all teachers? Are all workers of miracles? Have all the gift of healing? Do all speak with tongues? Do all interpret?” This man has one gift, that man another; and what that man has, this has not: there is a measure, a certain division of gifts. To men, therefore, it is given by measure, and concord among them makes one body. As the hand receives one kind of gift to work, the eye another to see, the ear another to hear, the foot another to walk; nevertheless the soul that does all is one, in the hand to work, in the foot to walk, in the ear to hear, in the eye to see; so are also the gifts of believers diverse, distributed to them as to members, to each according to his proper measure. But Christ, who gives, receives not by measure.
Source: Tractates on the Gospel of John (New Advent)