Ecclesiology
Who is the Church?  Is it a visible or earthly corporation -- a "church" in the sense of a specific denomination or institution, for instance?  Or is it the body of all believing Christians regardless of their denominational differences and disunity?  What is the relationship between living Christians and departed Christians (the "cloud of witnesses" Epistle to the Hebrews 12:1)-- do they (those on Earth and those in Heaven) constitute together the Church?