In the new system that Christ introduced, man belonged to the State by only a part of himself. Such a duality, however, bore within it the seeds of inevitable and recurring conflict. Two imperious forces shared the innermost loyalties of one and the same human being who had to work out his other-worldly salvation while at the same time fulfilling his earthly tasks as a member of the civic community.
Robert A. Graham, S.J., Vatican Diplomacy: A Study of Church and State on the International Plane (Princeton University Press, 1959)