Charity was also a favorite virtue with the beloved disciple. He frequently mentions it in his epistles, with the highest praise and commendation. And not only in his writings but in his discourses did he display the same devotedness to this virtue. So frequently did he repeat to his disciples the touching words, "My little children, love one another," that at last, as St. Jerome tells us, they became somewhat weary of always hearing the same, and asked him: Good master, why do you always give us this one command? His answer, says St. Jerome, was worthy of John; "Because it is the command of the Lord; and if you do this alone it will suffice."
— The Sinner’s Guide, Venerable Louis of Granada, man’s duty towards his neighbour