And his "Letter from a Birmingham Jail" is still one of the great pieces of American Christian ethics. But I want to quote from what might be a lesser known piece of his, "Paul's Letter to American Christians."
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But America, as I look at you from afar, I wonder whether your moral and spiritual progress has been commensurate with your scientific progress. It seems to me that your moral progress lags behind your scientific progress. Your poet Thoreau used to talk about "improved means to an unimproved end." How often this is true. You have allowed the material means by which you live to outdistance the spiritual ends for which you live. You have allowed your mentality to outrun your morality. You have allowed your civilization to outdistance your culture. Through your scientific genius you have made of the world a neighborhood, but through your moral and spiritual genius you have failed to make of it a brotherhood. So America, I would urge you to keep your moral advances abreast with your scientific advances.
I am impelled to write you concerning the responsibilities laid upon you to live as Christians in the midst of an unChristian world. That is what I had to do. That is what every Christian has to do. But I understand that there are many Christians in America who give their ultimate allegiance to man-made systems and customs. They are afraid to be different. Their great concern is to be accepted socially. They live by some such principle as this: "everybody is doing it, so it must be alright." For so many of you Morality is merely group consensus. In your modern sociological lingo, the mores are accepted as the right ways. You have unconsciously come to believe that right is discovered by taking a sort of Gallup poll of the majority opinion. How many are giving their ultimate allegiance to this way.
But American Christians, I must say to you as I said to the Roman Christians years ago, "Be not conformed to this world, but be ye transformed by the renewing of your mind." Or, as I said to the Phillipian Christians, "Ye are a colony of heaven." This means that although you live in the colony of time, your ultimate allegiance is to the empire of eternity. You have a dual citizenry. You live both in time and eternity; both in heaven and earth. Therefore, your ultimate allegiance is not to the government, not to the state, not to nation, not to any man-made institution. The Christian owes his ultimate allegiance to God, and if any earthly institution conflicts with God's will it is your Christian duty to take a stand against it. You must never allow the transitory evanescent demands of man-made institutions to take precedence over the eternal demands of the Almighty God.