I agree with Jody's point about the seige mentality of the institutional church, and the need to work both inside and outside the sanctuary - if I followed the argument correctly. But I'm inclined to play the part of the pedant and dispute the reading of Matt.16:18 ('upon this rock I will build my church, and the gates of hades will not [prevail against or withstand] it').

1. “Gates of Hades (pulai hadou)” looks to me like a semitic idiom for death or proximity to death. For example: ‘…[the Jewish martyrs] cried out in a very loud voice, imploring the Ruler over every power to manifest himself and be merciful to them, as they stood now at the gates of death’ (pulais hadou)’ (3 Macc.5:51; cf. 3 Macc.6:31; Job 38:17; Ps.9:13; 107:18; Is.38:10; Wisd.16:13; Odes 11:10; Ps. Sol.16:2).

2. I think that katischuô with the genitive must have a dynamic sense: ie. 'prevail against' or 'overcome' rather than withstand (cf. Jer.15:18 in the Septuagint).

Jesus is saying, therefore, that the 'gates of hades will not overcome the church'. But his point is not that the church must be always on the defensive. I think what he is saying is that as the disciples undertake a mission that will bring them into conflict with hostile political and spiritual forces, that will put their lives at risk (cf. 24-27), they can be reassured that nothing, not even death, will ultimately overcome the community that is based on the confession of Jesus as the Christ. The statement is important, therefore, precisely because the church must put itself at risk by going out into the world. So maybe we're back where we started but by a slightly different route.

(If this sort of comment is out of line in a postmodern theology discussion, tell me, and I won't do it again.)