Head and Heart, a survey of the intersections of American religious life with American political life from colonial times to the present, is far from uninteresting, but to my mind Wills has been more interesting about the same subjects elsewhere, especially in Under God. That book was focused more on its own moment (roughly, the election of 1988) than on the past, but Wills skillfully folded in enough history to give that book almost as much historical depth as this one has, so for a longtime Wills reader Head and Heart often gives one a feeling of déjà lu.
The opposition signalled in the title is between the United States as liberal, Enlightenment undertaking, religiously congruent with the deism of the founders or Emersonian Unitarianism or the mainline liberal Protestantism of Reinhold Niebuhr, and the United States as God's new chosen people, religiously congruent with the New England Puritans, the Great Awakenings, the current religious right. Wills, drawing with broad strokes, portrays the former as intelligent but dry and brittle, the later as juicily passionate but dim and bigoted. Typically the two tendencies are at odds, but when they ally -- as he argues they did in the abolition movement and the civil rights movement -- the U.S. lives up to its conception of itself.
Wills then is all for the occasional bracing infusion of religion into public life and debate, we might say, but he is also and crucially all for separation of church and state, so the last section of the book is a gloves-off drubbing of the Bush administration's trampling of that principle, and a fully deserved one. The richest part of the book, I'd say, if like me you happen to have already read Under God and Wills's books on 18th and 19th century American politics.