Calvin's Commentaries, Vol. 45: Catholic Epistles, tr. by John King, [1847-50], at sacred-texts.com
This Epistle is altogether worthy of the spirit of that disciple who, above others, was loved by Christ, that he might exhibit him as a friend to us. But it contains doctrines mixed with exhortations; for he speaks of the eternal Deity of Christ, and at the same time of the incomparable grace which he brought with him when he appeared in the world, and generally of all his blessings; and he especially commends and extols the inestimable grace of divine adoption.
On these truths he grounds his exhortations; and at one time he admonishes us in general to lead a pious and holy life, and at another time he expressly enjoins love. But he does none of these things in a regular order; for he everywhere mixes teaching with exhortation. But he particularly urges brotherly love: he also briefly touches on other things, such as to beware of impostors, and similar things. But each particular shall be noticed in its own place.