Mr. Mercer's points on what defines a church are poignant. He gently challenges the notion that church is about people of like age/stage, socio-economic status, affinity, or core values gathering together. In many ways, his post is simply a more pastoral version of the same challenge that Dr. Anthony Bradley is regularly putting before the PCA.
I love Mr. Mercer's closing points, which so neatly summarize what is and what isn't important in the identity of a church:
God is creating a community–a cross-cultural community—that is, a people that consists of folks who may be very different from one another, but who share a “common unity” in Jesus Christ. That is the NT vision of church. Our unity does not consist in the fact that we all have tattoos or like grunge music or meet in a pub. Nor does it consist in the fact that we are mostly conservative middle-class suburbanites. Nor does it consist in our whiteness or blackness or the specific ethnic culture in which we live. Nor is it about organs, hymns, robes, and pews.
Our only true oneness is in Christ. We accommodate to “where people are” to reach them in the world for Christ, making them disciples. But then, when we baptize them and teach them to observe all that Christ commanded us, we call them into the practice of cross-cultural love within the new family God is creating. What the world needs to see is faith communities made up of people vastly different from one another who have laid hold of that.
I highly recommend this post from Internet Monk, "Is It A Church?"