Following are excerpts from an article I wrote for people considering membership at the church I attend that are, I think, pertinent to this thread:
"In the Early Church described in Acts and in the letters of Paul, Peter and John, there was, in any particular locale, only one church. And so, if one wanted to be a properly-functioning member of the Church, the Body of Christ, one had to associate with one of these communities of the saints. There were no other options. If one lived in Ephesus and was part of the Ephesian church, leaving the church meant being completely cut-off from all Christian fellowship in Ephesus. There weren't dozens of other churches one could attend like there is today. Consequently, there was a natural strong involvement, born, in part, of a lack of options, that the first Christians had in their local body of fellow believers. There was also severe persecution of Christians that further pressed the earliest believers together. The effect of these factors upon New Testament followers of Christ was to unite them in very close-knit, highly-committed communities.
In contrast to the deeply-united fellowship of believers we read of in Acts (Acts 2:44-47), modern believers in North America have an increasingly fluid attachment to the church(es) they attend. In nearly every city, town or village, there is a plurality of churches from which one may choose. In the very consumerist culture of North America, this creates in Christians a “buffet-style” attitude toward church allegiance: If a church doesn't quite suit a believer, it is a simple matter to move to another church down the road that does. And if that church does not satisfy, well, one can just move on to another, and another, and another. As one might expect, this creates a rather migratory species of Christian that tends to maintain tenuous fidelity to any and all communities of believers.
Such light loyalty to any community of believers comes at a spiritual cost. God intends that there be a deep intimacy, a rich communion relationally and spiritually between believers (Jn. 13:34, 35; Ro. 12:5; 1 Pe. 1:22; 1 Jn. 1:7), that fosters holiness (2 Cor. 7:1; Ro. 6:19; 1 Thess. 3:13; 4:7), love, a thorough knowledge of, and commitment to, God's truth (Eph. 4:11-15), and a united and persistent effort by believers to fulfill the two great mandates God has set for the Church: Evangelism and the making of disciples (Mk. 16:15; Matt. 28:19). When Christians float about from church to church, never seriously committing themselves to a particular community of believers, none of these things is achieved with the same success or in the same measure as when Christians bind themselves to one another in a strongly-committed fellowship of believers."
And from farther on in the article:
"Paul the apostle wrote that “a little leaven leavens the whole lump.” (1 Cor. 5:6) That is, a little sin (leaven) in the life of an individual believer has a damaging effect spiritually upon the entire community of believers to which they belong. For this reason, accountability is extremely important for every local church body. ...accountability...assists the Church in being holy which is essential to fellowship with God, to obtaining His blessing, and to being properly useful to Him."