In this the children of God are manifest ... - 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 person is not truly a righteous person, 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 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 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 the notes at John 13:34-35.