I just started reading a little book called Church Membership: How the World Knows who Represents Jesus by Jonathan Leeman (Crossway 2012). The foreword by Michael Horton has a nice summary of how Christ's redemptive work relates to the visible church:
"Christ rules us in order to save us and saves us in order to rule us. Unlike the rulers of this age, Jesus doesn't ask us to shed our blood for his empire; he instead gave his own life for his realm. Then he was raised in glory as the beginning of the new creation, and now he is gathering coheirs into his kingdom who belong to each other because, together, they belong to him. The visible church is where you will find Christ's kingdom on earth, and to disregard the kingdom is to disregard the King" (15).
If the visible church is the place where the Kingdom of God becomes visible, then covenantal membership in a local church is instrumental for the visibility of the kingdom. A timely word in a day when church membership is often under-emphasized or left unmentioned altogether.