From Dr. Keller's latest blog post "Lloyd-Jones on the Practice of Real Preaching"...
I would argue that in a post-Christian culture, preaching will not be effective in the gathered assembly if Christians are not also highly effective in their scattered state. In our times, people will be indifferent or hostile to the idea of attending church services without positive contact with Christians living out their lives in love and service. Therefore the incarnational "dispersed" ministry of the church is extremely vital and necessary.