Matthew Poole's Commentary
Concerning the penman of the First Epistle, it doth not appear there hath been any doubt, the ancients generally ascribing it to the apostle St. John. The time of his writing it is uncertain, some assigning to it an earlier, others a later date. It is thought to have been written directly to the Christian Jews, not living in Judea, but remote in Parthia, (where it appears great numbers of them resided), being styled by a noted father, "The Epistle to the Parthians." The design of it is to confirm them in the great fundamental doctrine of Christianity: That our Lord Jesus was the Messiah, against the attempts of divers apostate or degenerate Christians, who (whether this Epistle were of the former or latter date) did in his time deny, or essentially deprave, that most important article. And not only to induce them all most stedfastly to believe it, but to impress it more deeply upon their souls, to reduce the more licentious, to raise and quicken the dead and carnal to a more strict, lively, vigorous Christianity; and (which is greatly inculcated) to excite and inflame mutual Christian love among them, as that which would more strongly fortify them against the endeavours of seducers, and render their communion more pleasant among themselves. The other two Epistles are very much of the same argument, (though the latter hath somewhat of a different and peculiar concernment), but doubted by some whether by the same penman, upon very insufficient grounds, the matter and style, plainly enough, showing them to be this apostle's.