The union of the churches broke before the union of the states. The Methodist Episcopal Church split north and south in 1844 over a slaveholding bishop, the Baptists followed in 1845 with the founding of the Southern Baptist Convention over whether a slaveholder could be a missionary, and the Presbyterians, who had already fractured on other lines, completed their sectional division in 1861. Each schism rehearsed the arguments the country would fight over, and each removed one more national institution that might have held the argument together.
Mark Noll has described what followed as a theological crisis. Both sides took the Bible as plain and authoritative, and on that shared ground the defenders of slavery had the easier case: the patriarchs held slaves, Paul returned Onesimus, and no verse abolishes the institution. Antislavery readers answered with the spirit of the whole against the letter of the part, with the golden rule and the Exodus, and with the argument that American slavery, racial and hereditary, was not the thing the ancient texts described. Neither side could defeat the other by exegesis, and a nation that had made scripture its common authority discovered it had no common way to read it.
What settled the question was not the argument but the war, a fact that Lincoln's second inaugural, with its two parties reading the same Bible and praying to the same God, stated more honestly than most sermons of either side.