Excellent article from Jonathan Parnell over at the Desiring God blog:
The founders of the United States of America were not trying to create a Christian nation in 1776. But they did view the Christian faith as beneficial to the type of republic they hoped to form. The man who led the way in articulating this benefit was John Witherspoon, a Presbyterian minister and influential founder who flies under the radar of most American history classes.
Witherspoon contended that the contribution of “true religion” to the public order is the morality of its people. If we were to translate this into our contemporary lingo we’d say, “The gospel’s influence on society is transformed lives.”
Witherspoon’s explanation is a piece of very applicable logic.
Biographer Jeffry H. Morrison gives a summarizing line and then quotes Witherspoon:
Religious leaders were to play a vital role, perhaps even more important than the laws themselves, in a republican society.
The return which is expected from them to the community is, that by the influence of their religious government, their people may be the more regular citizens, and the more useful members of society. I hope none here will deny, that the manners of the people in general are of the utmost moment to the stability of any civil society. When the body of a people are altogether corrupt in their manners, the government is ripe for dissolution.
Good laws may hold the rotten bark some longer together, but in a little time all laws must give way to the tide of popular opinion, and be laid prostrate under universal practice. Hence it clearly follows, that the teachers and rulers of every religious denomination are bound mutually to each other, and to the whole society, to watch over the manner of their several members [“Thanksgiving Sermon,” in Works, 5:265].
(John Witherspoon and the Founding of the American Republic, [Notre Dame, Indiana: University of Notre Dame, 2005], 23), paragraphing mine.