The greatest difference between present day Christianity and that of which we read in these letters is that to us it is primarily a performance, to them it was a real experience. We are apt to reduce the Christian religion to a code, or at best a rule of heart and life. To these men it is quite plainly the invasion of their lives by a new quality of life altogether. They do not hesitate to describe this as Christ "living in" them. Mere moral reformation will hardly explain the transformation adnd the exuberant vitality of these men's lives - even if we could prove a motive for such reformation, and certainly the world around offered little encouragement to the early Christians! We are practically driven to accept their own explanantion, which is that their little human lives had, through Christ, been linked up with the very Life of God.
J. B. Phillips, in Letters to Young Churches: A Translation of the New Testament Epistles.