In a letter to the Baptists of Danbury Connecticut in 1802, Thomas Jefferson assured them that “there will always be a wall of separation between Church and State”.
It was meant as encouragement for the baptists, who were scared of being persecuted by… the Congregationalists of Danbury Connecticut. Jefferson made it clear that the United States constitution enshrined secularism, and that they needn’t fear the potential of religious persecution.