Render the things of Caesar unto Caesar, and the things of God unto God
"Great temptation consists in the identification of Christianity with any sort of the kingdom of Caesar, i.e. in the enslavement of the infinite to the finite...
"Christianity is not revolutionary in the outward sense of the word. It has not entered into the world as a revolutionary social force, calling for a violent altering of the order of life. It is impossible even to call Christianity a force of social reform. The nature of Christianity is altogether inexpressible in the social categories of this world. Christianity has come into the world as the good news about salvation and about the Kingdom of God, which is not of this world. 'Seek ye first of all the Kingdom of God and His righteousness, and all this wilt be added unto you.''Be perfect, even as your Heavenly Father is perfect.' 'What doth it profit a man, if he gain all the entire world, but harm therein his soul.' 'For the Kingdom of God cometh not in perceived form, wherefore say not: lo, here be it, or lo, there be it. For the Kingdom of God is within you.' 'My Kingdom is not of this world.' A social revolution is utterly at odds with the words of Christ. Social revolution seeks first of all that which is 'to be added unto you,' and not the Kingdom of God; the makers of social revolution do not seek a perfection, like the perfection of the Heavenly Father; they want to gain the whole world—and by this they corrupt their own souls. The social revolution seeks for an order of life that will come in perceptible form, and about which it can be said, 'Lo, here it is' or 'Lo, there it is.' The kingdom to which the social revolution strives is 'of this world'. The same also can be said about the spirit opposite the revolution: imperialism..."
"Christianity cannot be wholly with the rightist camp, the leftist camp, or the centrist camp, since in all these camps there can be the same triumph of the godless kingdom of Caesar."
"It is impossible to realize the Kingdom of God by force. Not only human beings but even God cannot force grace. The freedom of man enters into the design of God for the Kingdom of God... The Church of Christ in this world always was and will be oppressed—either by a false protection, converting it into a tool of the state, to Caesar's ends, or by persecution."
"The realization by force of the Kingdom of God within the kingdom of Caesar is shown to be impossible. The kingdom of Caesar lives according to its own laws. And this catastrophic process, which is finishing off the modern era, is not only a matter of woe for the Church of Christ but also of rejoicing, since Christianity loses in quantity, but wins out in quality; truthfulness and sincerity triumph, and the lie and insincerity are struck down."
"It is possible to frighten Christians with persecutions, though in them it will build up a religious fortitude, while official protection, depriving the Church of its independence, can only enervate and paralyze the energy of Christians..."
"The kingdom of Caesar exists only in the natural, the non-transfigured world. Categories of might and authority, which derive entirely from the natural kingdom of Caesar, are inapplicable to the Kingdom of God. In it, everything is otherwise and dissimilar to our world and its laws. A theocratic and sacred autocratic monarchy will never again arise in the world. The holy Russian tsardom was the last of its type. This period in the history of Christianity has irreparably ended. And the visionary dream about its return is an harmful utopian and romantic dream; it is the lack of desire or the incapacity to stand before the ultimate religious realities. The Church knows only one Bridegroom—Christ. The Kingdom of God knows only one King—Christ."
"The mixed-up kingdom, in which 'the things of God' and 'the things of Caesar' were not sufficiently separated, wherein one was substituted for the other, has ended. The Christian state also was a jumbled half-Christian state. Now, half-fast Christianity is an impossibility. A time of choosing has begun. Christianity can be only a qualitatively inward, spiritual power in the world, and not a quantitative, outwardly coercive power. Really, Christianity can only be a power realizing the truth of Christ. The new wine is being brought forth in the Christian world and it is impossible to pour it into the old wine-skins. In the 'world' itself creative religious processes are being discovered, which ought to be recognized as churchly. But the third period, into which we enter, is not yet the final period. We live with the great hope, that there will begin a yet conclusive period in which will be manifest the miraculous power of the truth of Christ in the world, a power rising to life eternal, and that the Kingdom of God will come. The Church is not yet the Kingdom of God. The Kingdom of God comes imperceptibly not only within the visible protective-walls of the Church, but also into the world, into social and cosmic life, which are yet not perceived as churchly life. In the Kingdom of God there will be nothing of a resemblance to the kingdom of Caesar, to the present order of the natural world, it will be a real transfiguration of the cosmos, a new heaven and a new earth."
excerpted from this 1925 essay: "The Kingdom of God and the Kingdom of Caesar" (lightly reworked for readability)