Free exercise clause
The free clause exercise, along with the Establishment clause state "Congress shall make no law respecting an establishment of religion, or prohibiting the free exercise thereof...". Commonly known as the "religion clauses" from the first amendment, they prohibit 2 things: 1: the establishment of a national religion by Congress. 2: the preference of one religion over another. These clauses were made to keep religion and state seperate but have been largely ignored in the past.
Everson vs. Board of Education
In the 1900's, with a court case in 1947, the american government was scrutized over activity involving religious institutions and it was enforced as law that the government were not allowed to aid a religious organisation in any way financially.
Associate justice Hugo Black stated in conclusion:
The "establishment of religion" clause of the First Amendment means at least this: Neither a state nor the federal government can set up a church. Neither can pass laws which aid one religion, aid all religions, or prefer one religion over another. Neither can force nor influence a person to go to or to remain away from church against his will or force him to profess a belief or disbelief in any religion. No person can be punished for entertaining or professing religious beliefs or disbeliefs, for church attendance or non-attendance. No tax in any amount, large or small, can be levied to support any religious activities or institutions, whatever they may be called, or whatever form they may adopt to teach or practice religion. Neither a state nor the Federal Government can, openly or secretly, participate in the affairs of any religious organizations or groups and vice versa. In the words of Jefferson, the clause against establishment of religion by law was intended to erect "a wall of separation between church and State."