An excerpt from Letter from Birmingham Jail:
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. Segregation, to use the terminology of the Jewish philosopher Martin Buber, substitutes an “I-it” relationship for an “I-thou” relationship and ends up relegating persons to the status of things. 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. (emphasis mine)
Now I know there are some who reject the idea of natural law, but Aquinas puts forward a convincing argument with regard to the relationship between human and natural law, and to which we ultimately owe our obedience.
Thomas Aquinas’ treatment of the human law in his Summa Theologicae can be found here at the New Advent website. In fact, the entire treatment of law is an excellent read, especially when Aquinas talk about custom obtaining the force of law. Good stuff.