“One may well ask, ‘How can you advocate breaking some laws and obeying others?’ The answer is found in the fact that there are two types of laws: there are just laws, and there are 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 when 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 and natural law.” Martin Luther King, Jr. “Letter from a Birmingham Jail,” (August 1963), p. 3. http://web.cn.edu/kwheeler/documents/Letter_Birmingham_Jail.pdf
Rev. Dr. Martin Luther King, Jr. wrote this letter to answer critics of the non-violent protests against Birmingham’s segregation laws. He adhered to a long theological and legal tradition to justify violating unjust laws (in this case a court injunction against protests and demonstrations.) In the late fourth century Augustine had written: “For a law that is unjust does not seem to me to be a law at all.” (On Free Will I. 5. 11. in Augustine: Earlier Writings, trans. John H. S. Burleigh (Philadelphia 1953), p. 118.
If this is so, how do we determine if a law is just or not just? Later in the same work Augustine stated “…there is nothing just or legitimate in temporal law save what men have derived from eternal law.” On Free Will I. 6. 15. in Ibid., p. 121. Rev. King followed Augustine here too when he states that human law must agree with the moral law, or law of God. This leads us to King’s reference to Thomas Aquinas’s Summa Theologica. Aquinas, a 13th-century scholastic theologian, wrote “…laws may be unjust through being opposed to the Divine good: such are the laws of tyrants inducing to idolatry, or to anything else contrary to the Divine law: and laws of this kind must nowise be observed, because, as stated in Acts 5:29, ‘we ought to obey God rather than man.’ ” http://www.newadvent.org/summa/2096.htm#article4