The gospel creates community.
Because it points us to the One who died for his enemies, it creates relationships of service rather than selfishness. Because it removes both fear and pride, people get along inside the church who could never get along outside. Because it calls us to holiness, the people of God are in loving bonds of mutual accountability and discipline. Thus the gospel creates a human community radically different from any society around it.
Accordingly, the chief way in which we should disciple people (or, if you prefer, to form them spiritually) is through community. Growth in grace, wisdom, and character does not happen primarily in classes and instruction, through large worship gatherings, or even in solitude. Most often, growth happens through deep relationships and in communities where the implications of the gospel are worked out cognitively and worked out practically-in ways no other setting or venue can afford. The essence of becoming a disciple is, to put it colloquially, becoming like the people we hang out the most. Just as the single most formative experience in our lives is our membership in a nuclear family, so the main way we grow in grace and holiness is through deep involvement in the family of God. Christian community is more than just a supportive fellowship; it is an alternative society. And it is through this alternate human society that God shapes us into who and what we are. -Tim Keller, Center Church: Doing Balanced, Gospel-Centered Ministry in Your City