The Body, Charles W. Colson, 1992, Word Publishing, Page 70
Yet membership in a confessing body is fundamental to the faithful Christian life. Failure to do so defies the explicit warning not to forsake "our assembling together." His understanding of this prompted Martin Luther to say, "Apart from the church, salvation is impossible." Not that the church provides salvation; God does. But because the "saved" one can't fulfill what it means to be a Christian apart from the church, membership becomes the indispensable mark of salvation.
"So highly does the Lord esteem the communion of His church," Calvin wrote," that He considers everyone a traitor and apostate from religion who perversely withdraws himself from any Christian society which preserves the true ministry of the word and sacraments."