John Gill emphasizes the significance of loving one's brother in Christ as a reflection of being in the light of grace. He explains that true love is sincere and serves both temporal and spiritual needs, demonstrating that love is an evidence of one's relationship with God. A person who loves their brother avoids causing offense and strives to live in a way that does not lead others to stumble. This love, rooted in the light, leads to a life free from malice and sin, resulting in peace and joy in the community of believers.

Ver. 10. 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 in it; for though it is not his love to the brethren which is the cause of his light, of his being and continuing in it, for that is owing to the spirit of light and knowledge, but on the contrary, light is the cause of his love; yet it is an evidence of it, that by which it is known, as the cause is known by the effect; see Joh 3:14; and there is none occasion of stumbling in him, or "there is no scandal" or "offence in him"; he gives no offence to his brother, or at least, as much as in him lies, he takes care that he gives none; he avoids, as much as can be, putting a stumblingblock, or an occasion to fall, in his brother's way, by the use of things indifferent, or by any other action; nor will he easily take offence at what is said or done unto him, for charity or love is not easily provoked, it suffers long, and bears all things; see 1Co 13:4; nor does he so much and so frequently transgress the laws of God, and particularly those which regard his neighbour or his brother, and so easily fall into the snares of Satan, because he is in the light, and walks in the light, and sees his way, and what lies in his way, and, so shuns and avoids occasion of stumbling and falling. There is not in him that wrath, and malice, and envy, which lead on to the commission of other sins; for love works no ill, but fulfils the law, and will not suffer him to commit adultery, to kill, to steal, or bear false witness against his neighbour, friend, and brother; see Ro 13:9; and such an one enjoys great peace, tranquillity, and happiness; he has much comfort in himself, and pleasure in the saints, and delight in their company; he walks inoffensively, and in an harmless manner, without hurting himself, or any other, Ps 119:165.