Philip Ryken’s The Communion of Saints (Page 55):

“The same might be said of regular attenders who never join the church. They lack an unbreakable commitment to the church and its ministry. Nonmembers, however active they may be in the life of the church, are outside the covenant relationship with the body of Christ that God requires. They reserve the right to pick and choose their doctrine, lifestyle, and ministry. In effect they become their own elders denying the authority of the church to carry out its mandate of gathering and perfecting the saints. To put this in theological terms, they separate union with Christ, the head of the church, from union with his body. As a result, they confuse themselves and others – outside as well as inside the church – about what it means to be a Christian. This is a costly mistake to make because membership has its privileges. Martyn Lloyd-Jones went so far as to describe church membership as ‘the biggest honour which can come a man’s way in this world.’ There is no union with Christ apart from the communion of the saints. Nor can the saints have true communion without belonging to one another by belonging to Christ in his church. The communion of the saints is for members only.”

(HT: Oversight of Souls)