John Calvin on Hebrews 13:1

1. Let brotherly love, etc. Probably he gave this command respecting brotherly love, because a secret hatred arising from the haughtiness of the Jews was threatening to rend the Churches. But still this precept is generally very needful, for nothing flows away so easily as love; when everyone thinks of himself more than he ought, he will allow to others less than he ought; and then many offenses happen daily which cause separations. “Continue” or remain, implies that they had manifested this love, chapter 6:10; as though he had said, “Let the love of the brethren be such as it has been.” — Ed.

He calls love brotherly, not only to teach us that we ought to be mutually united together by a peculiar and an inward feeling of love, but also that we may remember that we cannot be Christians without being brethren; for he speaks of the love which the household of faith ought to cultivate one towards another inasmuch as the Lord has bound them closer together by the common bond of adoption. It was therefore a good custom in the primitive Church for Christians to call one another brothers; but now the name as well as the thing itself is become almost obsolete, except that the monks have appropriated to themselves the use of it when neglected by others, while at the same time they show by their discords and intestine factions that they are the children of the evil one.