The conceptual distinction between the exercise of authority and the exercise of power provides an essential guide to understanding the present and future status of Christendom, which has not been abolished but, rather, has taken on new forms in our times. The Second Vatican Council, in its document Lumen Gentium, clarified that the Kingdom of God is not a place or a government, much less an earthly end-state arrived at through the political process. Instead, it is “established by Christ as a communion of life, charity and truth; it is also used by him as an instrument for the redemption of all and is sent forth into the whole world as the light of the world and the salt of the earth.“ Its ecclesiastical manifestation is the pilgrim church that ”buds and grows until the time for the harvest” (cf. Mk. 4:26–29), while the perfected kingdom is found only in the final goal of the pilgrimage of the people of God.
Even if the distinction has not always been clear in practice throughout history, Christianity has consistently maintained a distinction between power and authority, between God and Caesar. This distinction has given rise to every manner of philosophical musing, most famously with Jean-Jacques Rousseau's claim that Christians cannot be good citizens. He said of the faith that it does not win the hearts of the faithful to the state but rather removes attachments to all earthly things. “I know nothing that is more actively opposed to the social spirit”—a spirit that he believed was achievable only through unanimous obedience to the state.
Rousseau was correct to observe that “Jesus came in order to set up a spiritual kingdom on earth; thereby the theological system was separated from the political system, and this in turn meant that the state ceased to be one state, and that inherent tensions emerged” between Christians and the goal of a unified state. Christians have never and will never completely submit to the goal of a unified civil and religious authority under centralized direction, nor are they inclined to favor one that works to their benefit, because the Kingdom of God can never be realized within political institutions.
What was new about Christianity was its unconditional claim that the believer belongs first to Christ (authority) and only second to the civil order (power). It is certainly true that at one time the church was an agent of persecution; at other times, society was an object of persecution by the church. Today, there is a constant and unending struggle to determine which is persecuting the other. But in all these debates, what emerges is a culture-wide consciousness that the two realms are indeed separate and distinct; there still exist a City of God and a City of Man, and they can live in peace, but they can never be conflated. The believer and the citizen may exist in the same person, but these remain two distinct roles. The civitas Dei and the civitas terrena remain unreconciled and will be so forever. When forced to choose, however, the believer must choose faith over the promise of unity in society or a politically imposed paradise on earth.