I frequently see in debates or arguments regarding the Separation of Church and State that it was meant to keep the State out of the Church, but not vice versa (because the founding fathers were Christian and intended this to be a Christian nation - both points which I know to be false, but that hardly matters to the issuant of the argument).

To be clear, I'm not looking to debate the specific point of Separation of Church and State; I am seeking to learn information about the nature of the entwinement of church and state that would have led the U.S. Founding Fathers to favor a separation. My question is generated from a debate point, but I am not trying to engage in that debate here. Sources or further reading would also be appreciated.