Albert Barnes Commentary 1 John 4:6

"We are of God: he that knoweth God heareth us; he who is not of God heareth us not. By this we know the spirit of truth, and the spirit of error." — 1 John 4:6 (ASV)

We are of God. John here, undoubtedly, refers to himself and to those who taught the same doctrines he did. He assumes that those to whom he wrote would admit this, and he argues from it as an indisputable truth. He had given them such evidence of this as to establish his character and claims beyond a doubt. Furthermore, he often refers to the fact that he was what he claimed to be, as a point so well established that no one would call it into question. See John 19:35; John 21:24; 3 John 1:12.

Paul, also, often refers to the same thing regarding himself: to the fact—a fact that no one would presume to call into question, and which might be regarded as the basis of an argument—that he and his fellow apostles were what they claimed to be. See 1 Corinthians 15:14–15; 1 Thessalonians 2:1–11.

Should not all Christians, and all Christian ministers, live in such a way that the same thing might be assumed about them in their interactions with their fellow men—that their characters for integrity and purity might be so clear that no one would be inclined to question them? There are such men in the church and in the ministry now; why might not all be such?

He that knoweth God, heareth us. Everyone who has a true acquaintance with the character of God will receive our doctrine. John could assume this, for he presumed it was undisputed that he was an apostle and a good man; and if this were admitted, it would follow that those who feared and loved God would receive what he taught.

Hereby. By this; that is, by the manner in which they receive the doctrines we have taught.

Know we the spirit of truth, and the spirit of error. We can distinguish those who embrace the truth from those who do not. Whatever claims to piety they might make, it was clear that if they did not embrace the doctrines taught by God's true apostles, they could not be regarded as his friends—that is, as true Christians.

It may be added that the same test is applicable now. Those who do not receive the plain doctrines laid down in the word of God, whatever pretensions they may make to piety, or whatever zeal they may display in the cause they have espoused, can have no well-founded claims to the name Christian. One of the clearest evidences of true piety is a readiness to receive all that God has taught. Compare Matthew 18:1-3; Mark 10:15; James 1:19–21.