Why Is One Covenant Not Enough?

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.