“A just law is a man-made code that squares with the moral law or the law of God. An unjust law is out of harmony with the moral law.”

It was with these very words, in his memorable Letter from the Birmingham Jail, that Martin Luther King, Jr., threw down the gauntlet in his great Civil Rights crusade. King refused to obey what he regarded as an immoral law that did not square with the law of God.

All across America today, millions of people are celebrating the birthday of this courageous man, and deservedly so. He was a fearless battler for truth, and all of us are in his debt because he remedied past wrongs and brought millions of Americans into the full riches of citizenship.

In schools and on courthouse steps, people will be quoting his “I Have a Dream” speech today. It is an elegant and powerful classic. But I would suggest that one of Dr. King’s greatest accomplishments, one which will be little mentioned today because it has suddenly become “politically incorrect,” is his advocacy of the true moral foundations of law.

King defended the transcendent source of the law’s authority. In doing so he took a conservative Christian view of law. In fact, he was perhaps the most eloquent advocate of this viewpoint in his time, as, interestingly, Justice Clarence Thomas may be today.

Writing from a jail cell, King declared that the code of justice is not man’s law: It is God’s law. Imagine a politician making such a comment today? We all remember the controversy that erupted weeks ago when George W. Bush made reference to his Christian faith in a televised national debate.

But the figure we celebrate today is a man who built his whole case on the argument, set forth by St. Augustine and Thomas Aquinas, that “An unjust law is no law at all.” To be just, King argued, our laws must always reflect God’s Law.

This is the great issue today in the public square: Is the law rooted in truth? Is it transcendent, immutable, and morally binding? Or is it, as liberal interpreters have suggested, simply what courts say it is? Do we discover the law, or do we create it?

Ever since Dr. King’s day, the United States Supreme Court has been moving us step-by-step away from the positions of this great Civil Rights leader. To continue in this direction, as I have written, can only lead to disastrous consequences—indeed, the loss of self-governing democracy.

So I would challenge each of us today to use this occasion to reflect not just on his great crusade for Civil Rights but also on his wisdom in bringing law back to its moral foundations.

Many think of King as some kind of liberal firebrand, waging war on traditional values. Nothing could be further from the truth. Dr. King was a great conservative who stood on the shoulders of Thomas Aquinas and St. Augustine, striving without apology to restore our heritage of justice.

This is a story I tell in my book, How Now Shall We Live?: a great moment in history when a courageous man applied the law of God to the unjust laws of our time, and made a difference. And that is the lesson we should teach our kids on this holiday. It is not just another day off from school or a day to go to the mall.

Read through King’s letter with you kids: It’s the most important civics lesson they’ll ever get.