John Chrysostom

"For this, Thou shall not commit adultery, Thou shall not kill, Thou shall not steal, Thou shall not bear false witness, and any other commandment, is briefly comprehended in this saying, Thou shalt love thy neighbor as thyself." He does not say merely it is fulfilled, but "it is briefly comprehended," that is, the whole work of the commandments is concisely and in a few words completed. For the beginning and the end of virtue is love. This it has for its root, this for its groundwork, this for its summit. …

Homily on Romans 23 ↗

Theophylact of Ohrid

He did not say "is completed," but "is summed up," that is, in this commandment the entire body of commandments is contained in summary. For the beginning and the end of virtue is love. Furthermore, the law requires love in the highest degree. "Love," it says, "your neighbor as yourself," but our Lord requires more, having taught that we should love our neighbor more than ourselves; for He teaches us to lay down our life for a friend (John 15:13).

Commentary on Romans ↗

Augustine of Hippo

No one loves his neighbor unless he loves God, and by loving him as himself, to the limit of his ability, he pours out his love on him so that he too may love God. But if he does not love God, he loves neither himself nor his neighbor.

Letter 20 ↗

Thomas Aquinas

Then when he says for you shall not commit adultery, he proves his proposition: first, by induction; second, by means of a syllogism, at the love of our neighbor. In regard to the first he proceeds inductively by enumerating certain precepts which fulfill the love of neighbor. And because the three precepts of the first tablet are more directly ordained to the love of God, he does not mention them, although they, too, are fulfilled in the love of neighbor, insofar as the love of God is included in the love of neighbor. …

Commentary on Romans ↗