Albert Barnes on Romans 12:9

Let love. The apostle proceeds to specify the duties of Christians in general, that they might secure the beauty and order of the church. The first which he specifies is love. This word here evidently refers to benevolence, or to good-will toward all mankind. In Rom 12:10, he specifies the duty of brotherly love; and there can be no doubt that he here refers to the benevolence which we ought to cherish towards all men, A similar distinction is found in 2Pet 1:7, "And to brotherly kindness add charity," i.e., benevolence, or good will, and kind feelings to others.

Without dissimulation. Without hypocrisy. Let it be sincere and unfeigned. Let it not consist in words or professions only, but let it be manifested in acts of kindness and in deeds of charity, 1Jn 3:18, Comp. 1Pet 1:22. Genuine benevolence is not that which merely professes attachment, but which is evinced by acts of kindness and affection.

Abhor that which is evil. The word abhor means, to hate; to turn from; to avoid. The word evil here has reference to malice, or unkindness, rather than to evil in general. The apostle is exhorting to love, or kindness; and between the direction to love all men, and the particular direction about brotherly love, he places this general direction to abhor that which is evil--that which is evil in relation to the subject under discussion, that is, malice or unkindness. The word evil is not unfrequently used, in this limited sense, to denote some particular or special evil, Mt 5:37,39, etc. Comp. Ps 34:14 2Ti 2:19 Ps 97:10 1Th 5:22.

Cleave to that which is good. The word rendered cleave to denotes, properly, the act of gluing, or uniting firmly by glue. It is then used to denote a very firm adherence to an object; to be firmly fitted to it. There it means, that Christians should be firmly attached to that which is good, and not separate or part from it. The good here referred to is particularly that which pertains to benevolence --to all men, and especially to Christians. It should not be occasional only, or irregular; but it should be constant, active, decided.

(v) "dissimulation" 1Pet 1:22 (w) "that which is evil" Ps 34:14