John, realizing that brotherly love was waning in the church, urged upon believers the constant need of this love. His letters to the church are full of this thought. "Beloved, let us love one another," he writes; "for love is of God; and everyone that loveth is born of God, and knoweth God. He that loveth not knoweth not God; for God is love. In this was manifested the love of God toward us, because that God sent His only-begotten Son into the world, that we might live through Him. Herein is love, not that we loved God, but that He loved us, and sent His Son to be the propitiation for our sins. Beloved, if God so loved us, we ought also to love one another."

Johannes, som var klar over, at broderkærligheden var ved at svinde hen indenfor menigheden, påmindede de troende om den stadige nødvendighed af at nære denne kærlighed. Hans breve til menighederne er fyldt af denne tanke. "I elskede, lad os elske hverandre," skriver han, "thi kærligheden er af Gud og enhver, der elsker, er født af Gud og kender Gud. Den, der ikke elsker, kender ikke Gud, thi Gud er kærlighed. Derved er Guds kærlighed blevet åbenbaret iblandt os, at Gud har sendt sin Søn, den enbårne, til verden, for at vi skal leve ved ham. Deri består kærligheden: ikke i, at vi har elsket Gud, men i, at han elskede os og sendte sin Søn til soning for vore synder. I elskede, når Gud således elskede os, så er også vi skyldige at elske hverandre."