Theological confusion, especially in matters which have to do with the Church, will inevitably produce consequences which are of grave practical concern. 

The identification of the Kingdom with the Church has led historically to ecclesiastical policies and programs which, even when not positively evil, have been far removed from the original simplicity of the New Testament ekklēsia. It is easy to claim that in the “present kingdom of grace” the rule of the saints is wholly “spiritual,” exerted only through moral principles and influence. But practically, once the Church becomes the Kingdom in any realistic theological sense, it is impossible to draw any clear line between principles and their implementation through political and social devises. For the logical implications of a present ecclesiastical kingdom are unmistakable, and historically have always led in only one direction, i.e., political control of the state by the Church. 

The distances down this road traveled by various religious movements, and the forms of control which were developed, have been widely different. The difference is very great between the Roman Catholic system and modern Protestant efforts to control the state; also between the ecclesiastical rule of Calvin in Geneva and the fanaticism of Münster and the English “fifth-monarchy.” But the basic assumption is always the same: The Church in some sense is the Kingdom, and therefore has a divine right to rule; or it is the business of the Church to “establish” fully the Kingdom of God among men. 

Thus the Church loses its “pilgrim” character and the sharp edge of its divinely commissioned “witness” is blunted. -  Alva J. McClain, The Greatness of the Kingdom: An Inductive Study of the Kingdom of God