John Calvin Commentary Hebrews 13:1

"Let love of the brethren continue." — Hebrews 13:1 (ASV)

Let brotherly love, et cetera. Probably he gave this command concerning brotherly love because a secret hatred, arising from the haughtiness of the Jews, was threatening to tear apart the Churches. But this precept is still generally very necessary, for nothing slips away so easily as love; when everyone thinks more of himself than he should, he will grant to others less than he should; and then many offenses happen daily that cause separations.

He calls love brotherly, not only to teach us that we should be mutually united by a special and inward feeling of love, but also so that we may remember that we cannot be Christians without being brothers. For he speaks of the love that the household of faith should cultivate toward one another, since the Lord has bound them more closely together by the common bond of adoption.

It was therefore a good custom in the primitive Church for Christians to call one another brothers. However, now the name, as well as the practice itself, has become almost obsolete.

The monks are an exception, having appropriated its use for themselves, though it is neglected by others. Yet, at the same time, they show by their internal discords and factions that they are the children of the evil one.