We are of God - John here, doubtless, refers to himself, and to those who taught the same doctrines which he did. He takes it for granted that those to whom he wrote would admit this, and 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; and he often refers to the fact that he was what he claimed to be, as a point which was so well established that no one would call it in question. See John 19:35; John 21:24; 3 John 1:12. Paul, also, not unfrequently refers to the same thing respecting himself; to the fact - a fact which no one would presume to call in 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. Might not, and ought not, all Christians, and all Christian ministers, so to live that the same thing might be assumed in regard to them in their contact with their fellow-men; that their characters for integrity and purity might be so clear that no one would be disposed to call them in question? There are such men in the church and in the ministry now; why might not all be such?
He that knoweth God, heareth us - Every one that has a true acquaintance with the character of God will receive our doctrine. John might assume this, for it was not doubted, he presumed, 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; to wit, by the manner in which they receive the doctrines which 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 pretensions they might set up for piety, it was clear that if they did not embrace the doctrines taught by the true apostles of God, they could not be regarded as his friends; that is, as true Christians. It may be added that the same test is applicable now. They 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 evince in the cause which 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.