We often think of today’s politics as especially fractious. But at least by the 1800 presidential election, America had already adopted no-holds-barred campaigns. In particular, the 1800 election between John Adams and Thomas Jefferson saw vitriolic attacks on Jefferson’s alleged atheism. (As I show in my forthcoming biography, Jefferson was a Unitarian—but certainly not an atheist.)

A pro-Adams newspaper repeatedly thundered that the question of the election was “GOD AND A RELIGIOUS PRESIDENT [Adams]” or “JEFFERSON—AND NO GOD.” Yes, they used ‘all caps’ before Twitter.

Adams was disposed toward Unitarianism, too, so his theology was not all that different from Jefferson’s. But the two differed sharply about whether state governments should continue to support a preferred Christian denomination, or an “establishment,” in the language of the First Amendment. Adams thought the states should keep established churches, and the established denomination in Massachusetts was the Congregationalist Church, the old church of the Puritans.

Jefferson, by contrast, delighted evangelicals—especially Baptists—by championing the disestablishment of the Anglican (or Episcopal) Church in Virginia. The process of disestablishment there concluded with the passage of Jefferson’s Bill for Establishing Religious Freedom in 1786.

Many of the Federalist pastors of New England, who received financial support from state governments, saw Jefferson as an existential threat to religion. They were wrong. Jefferson wanted to get the government out of the business of playing favorites in religion, but he hardly intended to launch an anti-Christian crusade like what France had seen in the darkest phases of their Revolution.

Jefferson’s friend Benjamin Rush gave a compelling biblical explanation for church-state separation in the midst of the fall 1800 election season, in a letter to Jefferson:

I agree with you in your wishes to keep religion and government independent of each Other. Were it possible for St. Paul to rise from his grave at the present juncture, he would say to the Clergy who are now so active in settling the political Affairs of the World. “Cease from your political labors, your kingdom is not of this World. Read my Epistles. In no part of them will you perceive me aiming to depose a pagan Emperor, or to place a Christian upon a throne. Christianity disdains to receive Support from human Governments.

From this, it derives its preeminence over all the religions that ever have, or ever Shall exist in the World. Human Governments may receive Support from Christianity but it must be only from the love of justice, and peace which it is calculated to produce in the minds of men. By promoting these, and all the Other Christian Virtues by your precepts, and example, you will much sooner overthrow errors of all kind, and establish our pure and holy religion in the World, than by aiming to produce by your preaching, or pamphlets any change in the political state of mankind.”

When pastors become political campaigners, politics can corrupt the church and distract it from its core business. Obviously, we never want the government to become hostile to religion, or to place believers under special disadvantages because of their faith. And we must remember that the First Amendment gives equal weight to disestablishment and to the free exercise of religion.

However, as a matter of prudence and the health of the church, we should never want church leaders to become partisan campaigners, regardless of the party in question. Getting involved in campaigning and partisanship disrupts the unity of the church, and risks turning the church into a servant of temporal power as much as the Kingdom of God.

Check out my new book Thomas Jefferson: A Biography of Spirit and Flesh (Yale University Press).