Whoever loves his brother abides in the light, and in him there is no cause for stumbling.
John says in the third chapter, that we falsely boast of love to God, except we love our brethren; and this is most true. But he now takes love to the brethren as a testimony by which we prove that we love God.
The seventh verse may be supposed either to look backward to what immediately preceded (and then it is walking as Christ walked that is here represented as no new, but an old commandment; it is that which the apostles would certainly inculcate wherever they brought Christ's gospel), or to look forward to what the apostle is now going to recommend, and that is the law...
Commenting on 1 John 2:7-11
He that loveth his brother,.... As such, and because he is his brother in Christ, and that cordially and sincerely, without hypocrisy and dissimulation, and by love serves him, both in things temporal and spiritual, and so observes the new, and yet old commandment, abideth in the light: it is a plain case, that such a man is in the light of grace, and continues...