The U. S. Founders believed in religious liberty and as Christians (or Christian-influenced), they knew that “Christianity Is the Mother of Political Liberty.” In fact, political liberty grew out of religious liberty in Europe. Therefore, we need to know what religious liberty means. It doesn’t mean that the state is neutral toward religion. The “Establishment clause” of the First Amendment had a very precise meaning in the late 18th century. It meant that the United States wouldn’t create an established national church as England did. Congress, therefore, was prohibited from establishing a national church (there was never any “Church of the United States”). Note this fact: The colonies, and then the states, almost all had either established churches or an establishment of a generic Christianity.
The Establishment clause
It’s hard for us to grasp this fact today, but the delegates to the Constitutional Convention demanded the Establishment clause not because they were secularists, but because they were Christians — they had their own state churches and religion and they didn’t want a national church to interfere with it. They didn’t want the Feds intruding into the state religion. The Establishment clause was never designed to de-sacralize culture or even the state.
The Free Exercise clause
Then there’s the free exercise clause. Congress may not prohibit the free exercise of religion. This clause obviously didn’t reflect a secularizing objective. The European religious settlements often had allowed religious persecution for dissenters. The Framers wanted to avoid this, and they wrote that avoidance into law. They weren’t secularists trying to keep Christianity out of the public square; they were Christians who didn’t want any particular church to suffer persecution for its beliefs and practices. They didn’t want the Presbyterians persecuting the Baptists, for example.
There’s no “separation of church and state” in the Constitution (it came from a private letter by Jefferson). However, properly understood, there is a legitimate “separation of church and state” implied in the Constitution — but not the separation of the state from God, which is another matter altogether. The Founders did not want a “Christian state” as such, but they did want a state that reflected certain Christian truths as they related to politics.
Persuasion, not coercion
Now, undergirding this view of religious liberty (which for the Founders implied liberty for Christians) is a striking rationale. The Founders believed that true religion — the Gospel of Christianity — doesn’t require the coerciveness of national politics to survive and thrive. They believed that if left in the free market of ideas, Christianity had nothing to fear. They were tired of the European idea that the nation had to be commandeered by one church, which then in turn got special privileges and protections. The Founders believed that genuine faith didn’t need coercive protections. All it needed was the freedom to propagate and proselytize.
Now, as you know, this is simply not true of many other religions — Islam, for example, has a very different church-state paradigm. Islam requires state protection and coercion precisely because it has no Gospel, no Good News. It’s a moralizing faith, not a redemptive faith. States shaped by Christianity can offer religious liberty to Islam, but true Islamic states like Iran cannot (consistently) allow full religious liberty to Christians or anybody else, except Muslims. Christians can afford religious liberty, but Islamists cannot.
Amid the present cultural depravity, today’s well-intentioned Christian calls for “post-liberalism” are actually suggestions to walk away from a consistently Christian social order. The expression for the generic Founding political philosophy is classical liberalism. It is the closest thing to a Christian political philosophy you can get. One of its cornerstones is religious liberty.
Religious liberty in the United States reflects a Christian conviction — that religion is a matter of persuasion, not coercion.
 On all of these points, see Daniel L. Dreisbach, “In Search of a Christian Commonwealth: An Examination of Selected Nineteenth-Century Commentaries on References to God and the Christian Religion in the United States Constitution,” Baylor Law Review, Vol. 48, No. 4 [Fall 1996], 928–1000.