Albert Barnes on 1 John 2:9

He that saith he is in the light. That he has true religion, or is a Christian. See 1Jn 1:7.

And hateth his brother. The word brother seems here to refer to those who professed the same religion. The word is indeed sometimes used in a larger sense, but the reference here appears to be to that which is properly brotherly love among Christians. Comp. Lucke, in loc.

Is in darkness even unto now. That is, he cannot have true religion unless he has love to the brethren. The command to love one another was one of the most solemn and earnest which Christ ever enjoined, (Jn 15:17;) he made it the peculiar badge of discipleship, or that by which his followers were to be everywhere known, (Jn 13:35;) and it is, therefore, impossible to have any true religion without love to those who are sincerely and truly his followers. If a man has not that, he is in deep darkness, whatever else he may have, on the whole subject of religion. Comp. 1Thes 4:9.

(e) "darkness" 2Pet 1:9