Many say that Martin Luther reformed our theology without significantly reforming the Constantinian church structure. Here is an interesting quote from Luther related by Roger Gehring in "House Church and Mission":
As early as 1526 Martin Luthere stressed the importance of the house church in his paper Deutsche Messe and Ordnung des Gottesdiensts on the "third kind" of worship service for those "who seriously want to be Christians and to proclaim the gospel with their actions and their words." These "would need to enter their name (in a list) and gather together (in a home) in a small group for prayer, to read (the Scripture), for baptism, to receive the sacraments, and to do other Christian services." But he regretted very much that he "did not yet have individuals who were doing this and did not see many who felt compelled to do so in the future."