First, then, let us notice that the Apostle speaks of these believers as "the epistle of Christ." He does not say the "epistles" but the "epistle," for he is not thinking simply of what is true of individuals, but of the whole company, though obviously, the company is composed of individuals.
Then let us remark, that the Apostle does not say, "Ye should be the epistle of Christ," but, "Ye are the epistle of Christ." Entertaining the wrong thought that we ought to be epistles of Christ, we shall set to work to become such by our own efforts. This would not only lead us into legal occupation with ourselves, but would also shut out the work of "the Spirit of the living God." The fact is that we become epistles of Christ, not by our own efforts, but by the Spirit of God writing Christ upon our hearts.
A Christian is one to whom Christ has become precious by a work of the Spirit in the heart. It is not simply a knowledge of Christ in the head, which an unconverted man may have, that constitutes a man a Christian, but Christ written in the heart. As sinners we discover our need of Christ, and are burdened with our sins. We find relief by discovering that Christ by His propitiatory work has died for our sins, and that God has set forth His acceptance of that work by seating Christ in the glory. We rest in God's satisfaction with Christ and His work, and our affections are drawn out to the One through whom we have been blessed. "Unto you therefore which believe He is precious." Thus Christ is written on our hearts and we become the epistle of Christ. If we are not the epistle of Christ we are not Christians at all.