From "Letter From a Birmingham Jail", April 16, 1963. Emphases mine:
One may well ask: "How can you advocate breaking some laws and obeying others?" The answer lies in the fact that there are two types of laws: just and unjust. I would be the first to advocate obeying just laws. One has not only a legal but a moral responsibility to obey just laws. Conversely, one has a moral responsibility to disobey unjust laws. I would agree with St. Augustine that "an unjust law is no law at all."
Now, what is the difference between the two? How does one determine whether a law is just or unjust? A just law is a man made code that squares with the moral law or the law of God. An unjust law is a code that is out of harmony with the moral law. To put it in the terms of St. Thomas Aquinas: An unjust law is a human law that is not rooted in eternal law and natural law.
There was a time when the church was very powerful--in the time when the early Christians rejoiced at being deemed worthy to suffer for what they believed. In those days the church was not merely a thermometer that recorded the ideas and principles of popular opinion; it was a thermostat that transformed the mores of society. Whenever the early Christians entered a town, the people in power became disturbed and immediately sought to convict the Christians for being "disturbers of the peace" and "outside agitators." But the Christians pressed on, in the conviction that they were "a colony of heaven," called to obey God rather than man. Small in number, they were big in commitment. They were too God-intoxicated to be "astronomically intimidated." By their effort and example they brought an end to such ancient evils as infanticide and gladiatorial contests. Things are different now. So often the contemporary church is a weak, ineffectual voice with an uncertain sound. So often it is an archdefender of the status quo. Far from being disturbed by the presence of the church, the power structure of the average community is consoled by the church's silent--and often even vocal--sanction of things as they are.
But the judgment of God is upon the church as never before. If today's church does not recapture the sacrificial spirit of the early church, it will lose its authenticity, forfeit the loyalty of millions, and be dismissed as an irrelevant social club with no meaning for the twentieth century.
Never before have I written so long a letter. I'm afraid it is much too long to take your precious time. I can assure you that it would have been much shorter if I had been writing from a comfortable desk, but what else can one do when he is alone in a narrow jail cell, other than write long letters, think long thoughts and pray long prayers?
Read his whole Letter from a Birmingham Jail, here.