John Calvin on Romans 12:9

9. Let love be, etc. Proceeding now to speak of particular duties, he fitly begins with love, which is the bond of perfection. And respecting this he enjoins what is especially necessary, that all disguises are to be cast aside, and that love is to arise from pure sincerity of mind. It is indeed difficult to express how ingenious almost all men are to pretend a love which they really have not, for they not only deceive others, but impose also on themselves, while they persuade themselves that those are not loved amiss by them, whom they not only neglect, but really slight. Hence Paul declares here, that love is no other but that which is free from all dissimulation: and any one may easily be a witness to himself, whether he has anything in the recesses of his heart which is opposed to love. “Love,” says an old author, “is the sum and substance of all virtues. Philosophers make justice the queen of virtues; but love is the mother of justice, for it renders to God and to our neighbor what is justly due to them.” — Ed. The words good and evil, which immediately follow in the context, have not here a general meaning; but evil is to be taken for that malicious wickedness by which an injury is done to men; and good for that kindness, by which help is rendered to them; and there is here an antithesis usual in Scripture, when vices are first forbidden and then virtues enjoined.

As to the participle, ἀποστυγούντες, I have followed neither Erasmus nor the old translators, who have rendered it “hating,” (odio habentes;) for in my judgment Paul intended to express something more; and the meaning of the term “turning away,” corresponds better with the opposite clause; for he not only bids us to exercise kindness, but even to cleave to it.