Albert Barnes on 1 John 3:10

In this the children of God are manifest, etc. That is, this furnishes 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 intimate that an important part of righteousness consists in brotherly love.

Whosoever doeth not righteousness, is not of God. In 1Jn 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 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 1Jn 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, Jn 13:34, Jn 13:35.