“There is no doubt that people who live apart from the assembled community can also belong to the sanctorum communio, namely the sick, or castaways for example…However, the assembly retains its full significance for the church-community. For those who belong to it also received their faith through concrete contacts with others, through ‘preaching’ (Rom. 10:17).
The assembly of believers remains our mother. Thus the question why, psychologically, one should stay with the assembly is first of all equivalent to the question why one should love one’s mother; if need be, one might answer by pointing to the motive of gratitude. The decisive factor, however, is that Christians will never feel they have outgrown the place of their spiritual birth. They thus seek the assembly not merely out of gratitude for the gift they have already received, but are driven by the desire to receive it ever anew, to be born anew again and again (John 3:3; 2 Cor. 4:16).
In all their solitude as individuals they know themselves to be members of the good shepherd’s flock; they know themselves to be part of the historical community of the church, the assembly, from which they have received life and continue to receive it, and in which alone they live. In the assembly the church community pledges itself to God, according to God’s will; and here God pledges to be present within the church-community.
The concrete assembly thus has a very specific significance of it’s own. First, it shows that church is something ‘visible’, a community of human beings consisting of body and soul; it is not a community of common convictions or based on kindred spirits, but a community of love made up of real human beings.”
Sanctorum Communio 228-229