Separation of church and state, as the original concept, was to restrict government intrusion in religious beliefs and practices. The exact words aren’t in the Constitution but are reflected in the “Establishment Clause” of the First Amendment.

“Congress shall make no law respecting an establishment of religion.”

The Framers were concerned with religious freedom, religious persecution, and self determination. This is the purpose of the Establishment Clause, to protect religion from secular government.

It was Thomas Jefferson, during his presidency, who used the precise words “a wall of separation between church and state” in a letter to (I think) Danbury Baptist Community. Jefferson’s letter was in response to Danbury Baptist Community’s concern of religious persecution as a minority religion.

The original purpose of the Establishment Clause was to keep secular government out of the affairs of religious belief and practice. Today, “separation of church and state” (not in the Constitution) is usually interpreted as keeping religious belief and practice out of secular government, such as prayer in public school and the Ten Commandments in government buildings. Modern interpretation of Thomas Jefferson’s words and the intent of the Establishment Clause are typically the opposite of the originally intended purpose.