John Calvin Commentary 1 John 3:10

"In this the children of God are manifest, and the children of the devil: whosoever doeth not righteousness is not of God, neither he that loveth not his brother." — 1 John 3:10 (ASV)

Whoever does not practice righteousness. To do righteousness and to do sin are here set in opposition to each other. Then, practicing righteousness is nothing other than to fear God from the heart and to walk in His commandments as far as human weakness will permit.

For though righteousness in a strict sense is a perfect keeping of the law, from which the faithful are always far off, yet as offenses and failings are not imputed to them by God, righteousness is that imperfect obedience which they render to Him. But John declares that all who do not live righteously are not of God, because all those whom God calls, He regenerates by His Spirit. Hence, newness of life is a perpetual evidence of divine adoption.

Neither he who does not love his brother. He accommodates a general doctrine to his own purpose. For until now he has been exhorting the faithful to brotherly love; now, for the same end, he refers to true righteousness. Hence this clause is added instead of an explanation.

But I have already stated the reason why the whole of righteousness is included in brotherly love. The love of God indeed holds the first place; but since love towards others depends on it, the love of God is often, as a part representing the whole, included under love towards others, while love towards others is also included under the love of God. Then he declares that everyone who is endowed with benevolence and humanity is thus just, and is to be regarded as such, because love is the fulfillment of the law. He confirms this declaration by saying that the faithful had been so taught from the beginning; for by these words he intimates that the statement which he made should not have seemed new to them.

In this the children of God are manifest. He concisely draws this conclusion: those claim a place and a name among the children of God in vain who do not prove themselves to be so by a pious and holy life, since by this evidence they show that they differ from the children of the devil. But he does not mean that they are manifested in this way, so as to be openly recognized by the whole world; rather, his meaning is simply this: that the fruit of their new life and their adoption always become evident in their lives.