The origin of the expression “separation of church and state” is found in a letter from Thomas Jefferson written to the Danbury Baptist Association in 1802. The Danbury Baptist Association had written a letter to the president voicing their concern that their state constitution lacked specific protections of religious freedom.
The Danbury Baptists wrote in the letter, “what religious privileges we enjoy (as a minor part of the State) we enjoy as favors granted, and not as inalienable rights. And these favors we receive at the expense of such degrading acknowledgments, as are inconsistent with the rights of freemen.”
Jefferson responded to the Danbury Baptists by referencing the Establishment Clause and Free Exercise Clause of the First Amendment.
“I contemplate with sovereign reverence that act of the whole American people which declared that their legislature should ‘make no law respecting an establishment of religion, or prohibiting the free exercise thereof,’ thus building a wall of separation between Church and State,” Jefferson said.
The metaphor of a “wall of separation” was not intended to say that religion should not influence opinion on government issues. Rather, it was used to affirm free religious practice for citizens.