John was a teacher of holiness, and in his letters to the church he laid down unerring rules for the conduct of Christians. "Every man that hath this hope in him," he wrote, "purifieth himself, even as He is pure." "He that saith he abideth in Him ought himself also so to walk, even as He walked." 1 John 3:3; 2:6. He taught that the Christian must be pure in heart and life. Never should he be satisfied with an empty profession. As God is holy in His sphere, so fallen man, through faith in Christ, is to be holy in his sphere.

Johannes blev en lærer i helliggørelse og i sine breve til menighederne opstillede han ufravigelige regler for, hvordan en kristen bør leve. "Enhver, der har dette håb til ham " skriver han, "renser sig selv, ligesom han er ren." Joh. 3,3. "Den, der siger, at han bliver i ham, han er også selv skyldig at vandre således, som han vandrede." Joh. 2,6. Han lærte, at en kristen må være ren i sit sind og sit liv. Han må aldrig lade sig nøje med tomme erklæringer. Ligesom Gud er hellig indenfor sit område, sådan skal det faldne menneske ved troen på Kristus være hellig indenfor sit område.