That’s the question blogger Camassia asks in an intriguing post about why the Baptist-style ecclesiology and voluntarism came to be a dominant form of religion in America:
When the Baptists came into being in England in the early 1600s, there were several church-state models around Europe: the international parastate that was Catholicism, the soft theocracy of the Church of England, the hard theocracy of Calvinism, the Two Kingdoms model of Luther, and the separatism of the Anabaptists. The Baptists were started by an Englishman who hung out with Anabaptists and adopted most of their beliefs, but with a few modifications. One of these was that, while Baptists believed in separation of church and state, they didnt think this meant total withdrawal from state affairs; Baptists could, and did, serve in the military and hold public office. The exact ramifications of this were just as fuzzy then as they are now. Some Baptists served Cromwell, for instance, hoping this would aid the cause of religious liberty, while others disagreed.
As unclear as the Baptist position was, this was more or less what the founding fathers enshrined in the Constitution. It presumed indeed, demanded robust citizen participation in government, and also preserved free exercise of religion, but forbade an established church. The fact that not all churches were equally prepared for this is apparent when you look at the dominant churches of the day . . . .
The reason why Baptist ecclesiology was so well suited to the U.S.A. is not just its views on church and state, but the underlying belief in the voluntary religious choices of individuals. This may sound like the same thing, but its actually somewhat different. The government in Augustines day, for instance, tolerated a number of local religions but granted religious authorities a lot of judicial power over their membership including, if need be, the power of the sword. By defending the rights of the individual to migrate from one church to another, the U.S. government is taking a position on the nature of church that favors a Baptist interpretation over others (certainly, over Augustines)