That principle is the total separation of religion and the State. 
Jesus in more than one place separated religion–the realm of God–entirely from the State–the realm of man. In one place: "Render to Cesar the things that are Cesar's, and to God the things that are God's." (Mark 12:17) In another place, "My kingdom is not of this world." (John 18:36)
When the gospel first went forth,–the everlasting gospel to be preached to every nation and to every creature,–it went forth under just such a state of things as is directly opposed to this divine principle.
When the gospel first went forth,–the everlasting gospel to be preached to every nation and to every creature,–it went forth under just such a state of things as is directly opposed to this divine principle.
Every man was compelled to be religious just as the Roman state said; and the state itself absorbed the individual with all his individuality. Yet the disciples of Christ preached everywhere in all that empire the principle that with religion or men's worship no government can of right have anything to do.
Finally came Protestantism announcing anew the principle to all the world that every man should worship as he pleased, and that religion should be separate from earthly government. Yet in not a single Protestant nation was the principle illustrated.
"The United States government not only announced to all the world a new order of things, but pledged itself forever to the new order of things, by placing on the national seal that declaration,"A New Order of Things," and "God has Favored the Undertaking."
Finally came Protestantism announcing anew the principle to all the world that every man should worship as he pleased, and that religion should be separate from earthly government. Yet in not a single Protestant nation was the principle illustrated.
"The United States government not only announced to all the world a new order of things, but pledged itself forever to the new order of things, by placing on the national seal that declaration,"A New Order of Things," and "God has Favored the Undertaking."
Of course God has favored the undertaking. 
Now this nation–the one of all the nations the most glorious, is the one that has been acknowledged as the enlightenment of all the nations, the one that has reached the highest place in the shortest time, of all that have ever been upon the earth.
Jefferson, Madison, and their noble fellow-workers for religious as well as civil freedom in this new nation, truthfully said:
"We hold it for a fundamental and undeniable truth that religion, or the duty which we owe to our Creator and the manner of discharging it,
can be directed only by reason and conviction,
not by force or violence.
The religion then of every man, must be left to the conviction and.jpg)
conscience of every man; and it is the right of every man to exercise it as these may dictate. This right is in its nature an unalienable right. It is the duty of every man to render to the Creator such homage, and such only, as he believes to be acceptable to him. We maintain, therefore, that in matters of religion no man's right is abridged by the institution of civil society, and that religion is wholly exempt from its cognizance."
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A.T. Jones