Therefore, lest this prove a stumbling-block to any, let us observe that in man government is twofold: the one spiritual, by which the conscience is trained to piety and divine worship; the other civil, by which the individual is instructed in those duties which, as men and citizens, we are bold to perform (see Book 4, chap. 10, sec. 3–6). To these two forms are commonly given the not inappropriate names of spiritual and temporal jurisdiction, but to the enacting of laws which require a man to live among his fellows purely honorably, and modestly. The former has its seat within the soul, the latter only regulates the external conduct. We may call the one the spiritual, the other the civil kingdom.
Now, these two, as we have divided them, are always to be viewed apart from each other. When the one is considered, we should call off our minds, and not allow them to think of the other. For there exists in man a kind of two worlds, over which different kings and different laws can preside. By attending to this distinction, we will not erroneously transfer the doctrine of the gospel concerning spiritual liberty to civil order, as if in regard to external government Christians were less subject to human laws, because their consciences are unbound before God, as if they were exempted from all carnal service, because in regard to the Spirit they are free. Again because even in those constitutions which seem to relate to the spiritual kingdom, there may be some delusion, it is necessary to distinguish between those which are to be held legitimate as being agreeable to the Word of God, and those, on the other hand, which ought to have no place among the pious.
Calvin, Institutes, 3.19.15