A tyrant must put on the appearance of uncommon devotion to religion. Subjects are less apprehensive of illegal treatment from a ruler whom they consider god-fearing and pious. On the other hand, they do less easily move against him, believing that he has the gods on his side.
When the government puts its imprimatur on a particular religion it conveys a message of exclusion to all those who do not adhere to the favored beliefs. A government cannot be premised on the belief that all persons are created equal when it asserts that God prefers some.
If I were a dictator, religion and state would be separate. I swear by my religion. I will die for it. But it is my personal affair. The state has nothing to do with it. The state would look after your secular welfare, health, communications, foreign relations, currency and so on, but not your or my religion. That is everybody's personal concern!
It strikes me often while I am in Iran that were Christian evangelicals to take a tour of Iran today, they might find it the model for an ideal society they seek in America. Replace Allah with God, Mohammad with Jesus, keep the same public and private notions of chastity, sin, salvation, and God's will, and a Christian Republic is born.
At some fundamental level, religion does not allow for compromise. It insists on the impossible. If God has spoken, then followers are expected to live up to God's edicts, regardless of the consequences. To base one's life on such uncompromising commitments may be sublime; to base our policy making on such commitments would be a dangerous thing.
The president is the high priest of what sociologist Robert Bellah calls the 'American civil religion.' The president must invoke the name of God (though not Jesus), glorify America's heroes and history,quote its sacred texts (the Declaration of Independence and the Constitution), and perform the transubstantiation of pluribus unum.