Namely that,

[T]here is no good reason [if God does not exist] to give special protection to religious liberties [as argued by] my former colleagues Brian Leiter ("Why Tolerate Religion?") and John Corvino ("Debating Religious Liberty and Discrimination"). Given their atheistic assumptions, they are quite right: James Madison’s arguments for the First Amendment explicitly presupposed the existence of God. In his "Memorial and Remonstrance against Religious Assessments" (1785), Madison wrote, “It is the duty of every man to render to the Creator such homage and such only as he believes to be acceptable to him. This duty is precedent… to the claims of Civil Society.” Our right to religious liberty rests logically on the reality of our supreme duty to God our Creator.