Religious toleration is a major tenet of American constitutionalism, but it presupposed other, more fundamental principles: agreement on natural rights and the ensuing rule of law. Richard Reeb elaborates the historical and philosophical context of religious toleration:
As Europe freed itself from the rule of theocratic regimes, dissenting religious denominations sought toleration from the domination of the most numerous or powerful sect, which generally was Roman Catholic in the south and Protestant in the north. [John] Locke's remarkable letter [on Toleration], which aimed at "mutual toleration of Christians in their different professions of religion," declares as a necessary condition "charity, meekness and good-will in general towards all mankind, even to those who are not Christian."
That is, all long as churches are "regulating... men's lives according to the rules of virtue and piety," religious tolerance is possible [emphasis added]. As difficult as it was for Christians at that time to admit it, Jews and Muslims can be tolerated as long as their loyalty is to the civil government under which they live, rather than to a foreign power. In Protestant countries, toleration of Roman Catholics seen as beholden to the Vatican was governed by the same principle....
In short, the necessary condition for Muslims' full participation in American citizenship is to renounce Sharia law. As long as any ambiguity or worse on this matter is tolerated, Muslims cannot be good citizens and they undermine American law. We can tolerate religious differences, but we cannot tolerate defiance of American law.