John Calvin on 1 John 4:19

19 We love him The verb ἀγαπῶμεν may be either in the indicative or imperative mood; but the former is the more suitable here, for the Apostle, as I think, repeats the preceding sentence, that as God has anticipated us by his free love, we ought to return to render love to him, for he immediately infers that he ought to be loved in men, or that the love we have for him ought to be manifested towards men. If, however, the imperative mood be preferred, the meaning would be nearly the same, that as God has freely loved us, we also ought now to love him.