Alan Knox at The Assembling of the Church asks why if we already have the New Covenant - which is ratified by the blood of Christ himself - do we need a church covenant to determine who is a member of the church and whether we should love those people and how. Of course, I have asked the same thing numerous times on this blog.
Alan frames the question in a slightly different way than I mostly have. Yes, good question. Why do we need Christ plus a man-made document? Christ's blood plus our signature? Give his short post a read.