William Burkitt on 2 John 1:5

Observe here, The duty required, together with the profession of the gospel; namely, to love in unity and peace, in amity and concord, one with another. This he calls a commandment, in the singular number, to intimate that in this one commandment all the rest are contained, and in keeping this we keep all: and he tells them, it is no new commandment, but an old one from the beginning; it is as old as Moses, yea, as old as Adam, being a part of the law of nature written in Adam's heart; yet, in some respects, love may be called a new commandment, because urged from new motives, and enforced by a new example; and because it is never to wax old, but to be always fresh in the memories, and found in the practice of Christ's disciples, to the end of the world: I write no new commandment, but what we had from the beginning, that we love one another.