In the High Middle Ages popes claimed authority to depose secular rulers; centuries later, troops of the French Revolution hauled Pope Pius VI out of Rome to stand trial (old and ill, he died en route in a provincial French town). The golden mean surely lies somewhere between these extremes.
And just here, in the U.S. at least, the two Religion Clauses of the First Amendment come into play.
The first says the government may not create an "establishment" of religion – the designation, that is, of a government-approved state church and, by extension, the direct support of religion precisely as religion. But the second, no less important than the first, says government may not inhibit free exercise of religion by individuals and groups.
Note that the first Religion Clause prohibits government from showing special favor to a particular church or even to religion in general. It does not forbid a friendly attitude by government toward religion or to religious groups that share the same vision of the common good that it holds. And – the key to the current debate about the evangelicals – it raises no objection to political activism by religious bodies exercising free exercise on behalf of their religious convictions.
But does a religious group have a right to get actively and directly involved in politics? And is that a good idea?
The answer to first question is: Almost certainly yes – religious groups have the same right to support and oppose policies and even politicians that non-religious groups have. That is implicit in the free exercise guarantee of the First Amendment. And Vatican Council II, speaking of the Catholic Church but using language that presumably would apply to any legitimate religious body, said religion should be free to "proclaim its teaching about society" and "pass moral judgment even in matters relating to politics, whenever the fundamental rights of man or the salvation of souls requires it" (Gaudium et Spes, 76).