In an incredible gaffe, Lady Christine repeated the age-old question that screams nescience. In fact, Ann Coulter even repeated this beaut, viz. that “the separation of church and state” doesn’t appear in the Constitution. True.
Thomas Jefferson in his letter to the Danbury Baptist Association in 1802 wrote in part the following. [Source]
Believing with you that religion is a matter which lies solely between Man & his God, that he owes account to none other for his faith or his worship, that the legitimate powers of government reach actions only, & not opinions, 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 & State.
That’s where the phrase comes from, Christine and Ann. That’s it. And I know you knew that.