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.
Source: Tractates on the Gospel of John (New Advent)