We speak what we know. Some refer this to Christ and John the Baptist; others say that the plural number is used instead of the singular. For my own part, I have no doubt that Christ mentions himself in connection with all the prophets of God, and speaks generally in the person of all. Philosophers and other vain-glorious teachers frequently bring forward trifles which they have themselves invented; but Christ claims it as peculiar to himself and all the servants of God, that they deliver no doctrine but what is certain. For God does not send ministers to prattle about things that are unknown or doubtful, but trains them in his school, that what they have learned from himself they may afterwards deliver to others. Again, as Christ, by this testimony, recommends to us the certainty of his doctrine, so he enjoins on all his ministers a law of modesty, not to put forward their own dreams or conjectures -- not to preach human inventions, which have no solidity in theme but to render a faithful and pure testimony to God. Let every man, therefore, see what the Lord has revealed to him, that no man may go beyond the bounds of his faith; and, lastly, that no man may allow himself to speak any thing but what he has heard from the Lord. It ought to be observed, likewise, that Christ here confirms his doctrine by an oath, that it may have full authority over us. You receive not our testimony. This is added, that the Gospel may lose nothing on account of the ingratitude of men. For since few persons are to be found who exercise faith in the truth of God, and since the truth is everywhere rejected by the world, we ought to defend it against contempt, that its majesty may not be held in less estimation, because the whole world despises it, and obscures it by impiety. Now though the meaning of the words be simple and one, still we must draw from this passage a twofold doctrine. The first is, that our faith in the Gospel may not be weakened, if it have few disciples on the earth; as if Christ had said, Though you do not receive my doctrine, it remains nevertheless certain and durable; for the unbelief of men will never prevent God from remaining always true. The other is, that they who, in the present day, disbelieve the Gospel, will not escape with impunity, since the truth of God is holy and sacred. We ought to be fortified with this shield, that we may persevere in obedience to the Gospel in opposition to the obstinacy of men. True indeed, we must hold by this principle, that our faith be founded on God. But when we have God as our security, we ought, like persons elevated above the heavens, boldly to tread the whole world under our feet, or regard it with lofty disdain, rather than allow the unbelief of any persons whatever to fill us with alarm. As to the complaint which Christ makes, that his testimony is not received, we learn from it, that the word of God has, in all ages, been distinguished by this peculiar feature, that they who believed it were few; for the expression -- you receive not -- belongs to the greater number, and almost to the whole body of the people. There is no reason, therefore, that we should now be discouraged, if the number of those who believe be small.

We speak - Jesus here speaks in the plural number, including himself and those engaged with him in preaching the gospel. Nicodemus had said John 3:2, "we know that thou art," etc., including himself and those with whom he acted. Jesus in reply said, we, who are engaged in spreading the new doctrines about which you have come to inquire, speak what we know. We do not deliver doctrines which we do not practically understand. This is a positive affirmation of Jesus, which he had a right to make about his new doctrine. he knew its truth, and those who came into his kingdom knew it also. We learn here:
1. That the Pharisees taught doctrines which they did not practically understand. They taught much truth Matthew 23:2, but they were deplorably ignorant of the plainest matters in their practical application.
2. Every minister of the gospel ought to be able to appeal to his own experience, and to say that he knows the truth which he is communicating to others.
3. Every Sunday school teacher should be able to say, "I Know what I am communicating; I have experienced what is meant by the new birth, and the love of God, and the religion which I am teaching."
Testify - Bear witness to.
That we have seen - Jesus had seen by his omniscient eye all the operations of the Spirit on the hearts of men. His ministers have seen its effects as we see the effects of the wind, and, having seen men changed from sin to holiness, they are qualified to bear witness to the truth and reality of the change. Every successful minister of the gospel thus becomes a witness of the saving power of the gospel.
Ye receive not - Ye Pharisees. Though we give evidence of the truth of the new religion; though miracles are performed, and proof is given that this doctrine came from heaven, yet you reject it.
Our witness - Our testimony. The evidence which is furnished by miracles and by the saving power of the gospel. Men reject revelation though it is attested by the strongest evidence, and though it is constantly producing the most desirable changes in the hearts and lives of men.

We speak that we do know - I and my disciples do not profess to teach a religion which we do not understand, nor exemplify in our conduct. A strong but delicate reproof to Nicodemus, who, though a master of Israel, did not understand the very rudiments of the doctrine of salvation. He was ignorant of the nature of the new birth. How wretched is the lot of that minister, who, while he professes to recommend the salvation of God to others, is all the while dealing in the meagre, unfruitful traffic of an unfelt truth! Let such either acquire the knowledge of the grace of God themselves, or cease to proclaim it.
Ye receive not our witness - It was deemed criminal among the Jews to question or depart from the authority of their teachers. Nicodemus grants that our Lord is a teacher come from God, and yet scruples to receive his testimony relative to the new birth, and the spiritual nature of the Messiah's kingdom.

Verily, verily, I say unto thee, We speak that we do know, and testify that we have seen; and ye receive not our (i) witness.
(i) You handle doubtful things even though you have no solid basis for believing them, and yet men believe you: but I teach those things that are of a truth and well known, and you do not believe me.

Verily, verily, I say unto thee, we speak that we do know,.... Meaning either himself, and John the Baptist his forerunner, who preached the same doctrine of regeneration, internal sanctification, and evangelical repentance, as well as outward reformation, as necessary to entrance into the kingdom of heaven, or the Gospel dispensation, he declared was just at hand; or his disciples with himself, who were now with him, and whom he had called to preach the same truths he himself did; or the prophets of the Old Testament, who agreed with him in these things; or the Father that was with him, and never left him alone, and the Holy Spirit that was upon him, by whom he was anointed to preach these things, and who spoke them in him; or else he may use the plural number of himself alone, as being one in authority, and speaking with it, as he sometimes did, Mark 4:30, and the rather this seems to be the sense, since he immediately, in the next verse, speaks in the singular number, "if I have told you earthly things", &c. Now Christ must needs thoroughly, and certainly know what he spoke, since he was not only the omniscient God, but, as Mediator, had all the treasures of wisdom and knowledge in him, and the spirit of wisdom and knowledge rested on him:
and testify that we have seen; and therefore ought to have been received as a credible witness, as he was a faithful one; since "seeing" and "knowing" are qualifications in a witness, Leviticus 5:1; and though these were eminently in Christ, the generality of the Jews gave no credit to his testimony:
and ye received not our witness; which was an aggravation of their sin and unbelief; see John 3:32.

We speak that we know, and . . . have seen--that is, by absolute knowledge and immediate vision of God, which "the only-begotten Son in the bosom of the Father" claims as exclusively His own (John 1:18). The "we" and "our" are here used, though Himself only is intended, in emphatic contrast, probably, with the opening words of Nicodemus, "Rabbi, we know.", &c.
ye receive not, &c.--referring to the class to which Nicodemus belonged, but from which he was beginning to be separated in spirit.

We speak that we do know . . . ye receive not our witness. "Ye" includes Nicodemus and all Jews who failed to confess him; "we" includes himself and those who should testify of him by the Spirit.

We speak what we know - I and all that believe in me.