From “Letter from a Birmingham Jail.”

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. Any law that uplifts human personality is just. Any law that degrades human personality is unjust. All segregation statutes are unjust because segregation distort the soul and damages the personality. It gives the segregator a false sense of superiority and the segregated a false sense of inferiority…. Hence segregation is not only politically, economically and sociologically unsound, it is morally wrong and awful. Paul Tillich said that sin is separation. Is not segregation an existential expression ‘of man’s tragic separation, his awful estrangement, his terrible sinfulness? Thus it is that I can urge men to obey the 1954 decision of the Supreme Court, for it is morally right; and I can urge them to disobey segregation ordinances, for they are morally wrong.