Anyone tempted by Rick Santorum's proposal to knock a hole in the wall between church and state ought to consider the old maxim: "Be careful what you wish for — you might get it."

On the GOP presidential campaign trail, the former senator from Pennsylvania excoriates those who think the First Amendment means what it says — that government should keep its hands off matters of faith. "The president has reached a new low in this country's history of oppressing religious freedom that we have never seen before," Santorum has said, a verdict that seemingly resonated with voters in Mississippi and Alabama, where he won the Republican primaries.

But Santorum has it topsy-turvy: Separation of church and state isn't the enemy of freedom of religion, but its best guarantee, and you don't have to take my word for it. Millions of Europeans rendered that judgment with their feet, leaving Old World homelands with established churches for this country's religious diversity. Almost nobody went the other way.

The Pilgrims we honor on Thanksgiving were "non-conformists," in 17th century jargon. They were dissidents who refused to fall in line behind their monarch's theology. So, too, were the Quakers and the Poles, who flocked to the U.S. to escape 19th century German overlords determined to wean them from their Catholic faith.

History books aren't the only place to learn what happens to freedom of conscience where there is no separation of church and state. Take a trip to Saudi Arabia, but don't wear a crucifix, and keep your Bible under wraps. Public display of non-Muslim religious articles is forbidden. If you're a woman, don't bother reserving a rental car at the King Khalid International Airport in Riyadh. Women can't drive there.

Presumably most Saudis don't feel those rules as restrictions, being virtually all Sunni Muslims. But our country hosts a patchwork quilt of religious ideas — like, for example, the concept of a messiah. Muslims don't believe in it, considering Muhammad the last of the prophets. Christians say Jesus was the messiah. Jews think he is yet to come.

So if a President Santorum (or anyone so inclined) were to take a whack at the wall between church and state, where would the crack come? At the Muslim, the Christian or the Jewish take on the messiah?

A similar problem is posed by the seemingly uncontrovertible Ten Commandments. Who could argue with "Thou shalt not steal"? But Anglicans and Jews say it's the eighth commandment; by Catholic and Lutheran count, it's the seventh. So if those judges who yearn to display the commandments were to get a pass, whose numbering system would get a courtroom seal of approval?

According to pollsters, a lot of Americans wish religion had a larger place in public life, making them a potential constituency for Santorum. Presumably, they share a nagging sense that politics shouldn't be just about winning elections. It should be informed by values and spiritual ideals, which is an understandable and admirable instinct. The only problem is there is no such thing as "religion," only myriad denominations and faith communities. Whose, then, sets the national agenda? Consider the hot-button religious issues of the current presidential primaries: artificial birth control and abortion.

Neither term occurs in the Bible. And in the Scripture's silence, theologians have had to search out clues and take hints from biblical narratives. Not surprisingly, they come to widely different conclusions. Many mainstream Protestants support what they see as a woman's right to choose. The Catholic Church rejects that argument. It traces life's sanctity to the moment of conception.

Given that irreconcilable divergence of belief, what would be our national policy, should the wall between church and state come down? A Santorum administration would push to overturn Roe v. Wade, Santorum being Catholic. Would it then be reinstated, if a President Santorum were to be succeeded by a liberal Protestant?

The world has seen what happens when rulers get to set religious rules, yet can't agree over the generations on what those rules should be. It ain't pretty.

Henry VIII unseparated church and state by having himself, not the pope, proclaimed head of the Church of England in 1534. Under his son, Edward VI, Protestantism was favored and Catholics persecuted. Edward's sister and successor, Mary, favored Catholicism, so Protestants were persecuted. During her reign, Elizabeth, Henry's other daughter, tried papering over religious differences by espousing a middle-of-the-road national faith. Yet in the 17th century, the struggle was renewed. Under suspicion of Catholicism, a king was executed; and a Protestant military dictatorship installed. Then the monarchy was restored, but a new rule added that rulers couldn't be Catholic.

It's little wonder that, in the midst of that turmoil, the Pilgrims and Puritans decided to take a chance on New England's frosty climate, rather than witness their homeland's next change of religious fortune.

Who could blame them, since they were essentially following the blueprint of the inventor of separation of church and state. He wasn't some secular humanist or egghead professor, but a simple country preacher, who lived 2,000 years ago.

"Render therefore unto Caesar the things that are Caesar's, and unto God the things that are God's," Jesus of Nazareth said, concisely anticipating the wondrous sentiment underlying the First Amendment to the U.S Constitution:

"Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof."

Ron Grossman is a Tribune reporter and former history professor.