But he who receiveth his testimony. Here he exhorts and encourages the godly to embrace boldly the doctrine of the Gospel, as if he had said that there was no reason why they should be ashamed or uneasy on account of their small number, since they have God as the Author of their faith, who alone abundantly supplies to us the place of all the rest. And, therefore, though the whole world should refuse or withhold faith in the Gospel, this ought not to prevent good men from giving their assent to God. They have something on which they may safely rest, when they know that to believe the Gospel is nothing else than to assent to the truths which God has revealed. Meanwhile, we learn that it is peculiar to faith to rely on God, and to be confirmed by his words; for there can be no assent, unless God have, first of all, come forward and spoken. By this doctrine faith is not only distinguished from all human inventions, but likewise from doubtful and wavering opinions; for it must correspond to the truth of God, which is free from all doubt, and therefore, as God cannot lie, it would be absurd that faith should waver. Fortified by this defense, whatever contrivances Satan may employ in his attempts to disturb and shake us, we shall always remain victorious. Hence, too, we are reminded how acceptable and precious a sacrifice in the sight of God faith is. As nothing is more dear to him than his truth, so we cannot render to him more acceptable worship than when we acknowledge by our faith that He is true, for then we ascribe that honor which truly belongs to him. On the other hand, we cannot offer to him a greater insult than not to believe the Gospel; for he cannot be deprived of his truth without taking away all his glory and majesty. His truth is in some sort closely linked with the Gospel, and it is his will that there it should be recognized. Unbelievers, therefore, as far as lies in their power, leave to God nothing whatever; not that their wickedness overthrows the faithfulness of God, but because they do not hesitate to charge God with falsehood. If we are not harder than stones, this lofty title by which faith is adorned ought to kindle in our minds the most ardent love of it; for how great is the honor which God confers on poor worthless men, when they, who by nature are nothing else than falsehood and vanity, are thought worthy of attesting by their signature the sacred truth of God?

He that hath received his testimony - Hath received and fully believed his doctrine. Hath yielded his heart to its influence.
Hath set to his seal - To "seal" an instrument is to make it sure; to acknowledge it as ours; to pledge our varacity that it is true and binding, as when a man seals a bond, a deed, or a will. Believing a doctrine, therefore, in the heart, is expressed by "sealing it," or by believing it we express our firm conviction that it is true, and that God who has spoken it is true. We vouch for the veracity of God, and assume as our own the proposition that it is the truth of God.
God is true - Is faithful; is the author of the system of doctrines, and will fulfill all that he has promised. We learn here:
1. that to be a true believer is something more than to hold a mere speculative belief of the truth.
2. that to be a believer is to "pledge ourselves" for the truth, to seal it as our own, to adopt it, to choose it, and solemnly assent to it, as a man does in regard to an instrument of writing that is to convey his property, or that is to dispose of it when he dies.
3. Every Christian is a witness for God, and it is his business to show by his life that he believes that God is true to his threatenings and to his promises. See the notes at Isaiah 43:10.
4. It is a solemn act to become a Christian. It is a surrender of all to God, or giving away body, soul, and spirit to him, with a belief that he is true, and alone is able to save.
5. The man that does not do this - that is not willing to pledge his belief that God is true, sets to his seal that God is a liar and unworthy of confidence, 1-John 5:10.

Hath set to his seal - That is hath hereby confirmed the truth of the testimony which he has borne; as a testator sets his seal to an instrument in order to confirm it, and such instrument is considered as fully confirmed by having the testator's seal affixed to it, so I, by taking up this testimony of Christ, and proclaiming it to the Jews, have fully confirmed it, as I know it to be a truth; which knowledge I have from the immediate inspiration of the Holy Spirit. See John 1:33, John 1:34.

He that hath received his testimony,.... For there was here and there one that did, who believed in him as the Messiah, and embraced his Gospel, and submitted to his ordinances, and truly and sincerely followed him: and for the encouragement of such, it is said,
hath set to his seal that God is true; faithful in fulfilling the promises he has made concerning the Messiah, and his coming: he firmly believes that God is true to every word of his, and will make good every promise; and this he seals, ratifies, and confirms by his embracing the testimony of Christ; whereas, on the contrary, he that believes not makes God a liar, than which, nothing can be more reproachful to him, 1-John 5:10. The Jews have a saying (z) that "the seal of the blessed God is truth". The Arabic version renders it, "he is already sealed, because God is true"; and the Ethiopic version, "God hath sealed him, because he is true"; namely, with his holy Spirit; see 2-Corinthians 1:22.
(z) T. Hieros. Sanhedrin, fol. 18. 1. & T. Bab. Sanhedrin, fol. 64. 1. & Yoma, fol. 69. 2.

hath set to His seal, &c.--gives glory to God whose words Christ speaks, not as prophets and apostles by a partial communication of the Spirit to them.

Hath set to his seal - It was customary among the Jews for the witness to set his seal to the testimony he had given. That God is true - Whose words the Messiah speaks.