They [regarding visible/invisible church distinction] are not properly two churches, but one church, contemplated in two different aspects — an internal and an external. They do not occupy different spheres, but the same sphere. The visible church includes or contains the invisible, though, in its present imperfect condition, it has also mixed up with it some inferior elements, — some chaff, which will one day be separated from the wheat.

William Cunningham, Historical Theology, Vol. 1, 16.