John Chrysostom
'Owe no one anything, except to love one another.' The Apostle again turns to the mother of all good things, the teacher of everything said above, the creator of every virtue. He says that love too is our debt, but not like tax or tribute. It is constant. He desires that this debt should never be paid off. Rather, it should always be paid, but not in full, so that we constantly remain in debt. For this is precisely the kind of debt that must always be paid and always remain due. Having said how one must love, the Apostle also points out the benefit of love, saying, 'For he who loves another has fulfilled the law.' Do not count this a favor, since this too is a debt. You are obliged to love your brother because of your spiritual kinship with him.
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Not only because of kinship, but also because we are members of one another. If love grows poor in us, everything is destroyed. Therefore love your brother. If from love toward him you gain the benefit of fulfilling the law, then you are obliged to love him also because you have been benefited by him.