Constitutional Religion

Was “religion” in the First Amendment implicitly understood to mean “Christian denominations” or “Christianity”?

Not by everyone. During the debates over disestablishment in Virginia in 1785, Jefferson explained that the vote on one amendment proved that “they meant to comprehend within the mantle of its protection the Jew and the Gentile, the Christian and Mahometan, the Hindu and Infidel of every denomination.” Jefferson certainly would have understood the First Amendment in the same fashion.