Albert Barnes 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)

In this the children of God are manifest, etc. That is, this provides a test of their true character. The test is found in doing righteousness and in the love of the brethren. The former he had illustrated; the latter he now proceeds to illustrate. The general idea is that if a man is not truly a righteous man and does not love the brethren, he cannot be a child of God. Perhaps by the phrase "in this," using a pronoun in the singular number, he means to suggest that an important part of righteousness consists in brotherly love.

Whosoever doeth not righteousness, is not of God. In 1 John 3:7, he had said that he that doeth righteousness is of God. If that is true, then what he here affirms must be true also: that a man who does not do righteousness is not of God. The general idea is the same, that no one can be a true Christian who is not in fact a righteous man.

Neither he that loveth not his brother. The illustration of this point continues to 1 John 3:18. The general sense is that brotherly love is essential to the Christian character, and that he who does not possess it cannot be a Christian. On the nature and importance of brotherly love as an evidence of piety, see Barnes on John 13:34 and John 13:35.