Fellowship is the instituted way of making a public profession of the faith and hope of the gospel.
A man may hold the opinions and approve the practices of some voluntary, worldly society; but until he has united himself with it, he is not considered, either by its members or the public, as one of their number. His actually joining himself to them according to the established usage, is his profession. Thus a man may be a sincere believer of the gospel, and, so far as respects his own private conduct, an exemplary example of genuine piety; but until he has connected himself with a Christian church, he has not professed himself to be a Christian. It is by that act he declares to the world his faith and hope as a believer in Christ. It is thus he virtually says, “I receive the opinions, possess the dispositions, submit to the obligations, and observe the practices of the church of God with which I now connect myself.” Jesus Christ has made it our duty not only to receive his truth into our hearts, but to confess him before men; and it is a duty on which very considerable stress is laid. This is to be done, not in any ostentatious way, but by joining ourselves to his people: which is a confession, that both the church and the world clearly understand. Hence it is apparent, that church membership is no trifling matter, since it is calling heaven, earth, and hell, to witness our solemn declaration of submission to the authority of Christ. It is saying, in the hearing of more worlds than one, “I am a Christian.”
–J. A. James, The Church Member’s Guide, pp. 27-28.