But when it comes to the content, this book is right on the border between "worthless" and "dangerous." Since I've already commented on some of the specifics (links at the end), I'll keep it general here.
Disclaimer (which I've given before): I am in no way questioning whether or not the authors of this book are Christian. I'm merely commenting on their political philosophy, interpretations of history, and theology.
In general, there are two fundamental problems with this book: its bad history and its bad theology.
I've already dealt with some of the more specific egregious historical errors, but in general Under God is problematic in how it lays out a two-sided approach to American history. On the one hand, we are given a vision of a golden past where America was God's nation and all the Founding Fathers were Christian. This past gradually declined into the present, where atheism has run rampant and we have gotten away from our roots as a Christian nation. Story after story of our Founders' faith (even the anti-Trinitarian John Adams is used at one point, along with the cultist Sojourner Truth) are thrown out before us along with out-of-context quotes, questionable interpretations, and frankly a one-sided view of history. It probably bears repeating that not only were most of the Founders NOT Christian, even the few that were could not agree on whether or not the American revolution was a good thing (to say nothing of the later Constitution--which Christians for the most part seemed to oppose).
We must remember that "The Founders" were not a monolithic entity whose spirit we need to try to recapture today, they were a diverse group of individuals and factions who each had their own views on religion, politics, and the world. I'll include some links below for good overview sources for the Founding Period that will give a better interpretation of the various views of the generation of the 1770s.
Alas, that's not the only problem. On the one hand we have the Golden Age view of American history. On the other hand, Under God gives us a vision of a past mired in the sin of slavery, racism, and sexism. Over time, so the book implies, we have gradually clawed our way out of this past into the enlightened present, where these horrible evils are dead and we have true brotherhood in our nation (or would have, if only we'd get back to our Christian roots). To be sure, the darker aspects of American history are relevant and do demand much more attention than they've been given (especially at the popular level), but I'm not convinced this is the way to do it. While most of the stories on this topic in Under God are by themselves unobjectionable and fairly straightforward (if not always completely historically accurate), as the book progresses a pattern begins to unfold--a pattern intended to suggest that America used to struggle with these evils, but no longer does. As a result, I believe these past-negative stories are little more than a cathartic "see, we believe in sin too--but fortunately it was all in the way back when, and we've moved beyond it." Which works if we're talking about slavery (the bulk of the book's attention, though Native Americans and suffrage get nods as well). Clearly we no longer have slaves--that is an evil that has been vanquished. And yet, if we start talking about greed, or apathy, or pride, or gluttony, or self-indulgence, or, well, any other of a host of sins that we could raise, it's hard to see how we can have any kind of rosy view of moral progress (or regress) in America.
But so what? It's a history book, shouldn't everything be about the past anyway? This is a big deal because Under God's treatment of sin in American history requires absolutely nothing of the reader other than a feeling of shame and regret, followed by a feeling of relief as we note that the things which cause our shame and regret are increasingly in the distant past. True, we are perhaps concerned at the atheistic tendencies of the present, and filled a longing for the golden days of yore when everyone was a Christian. And maybe we're even tempted to get angry at the decline of religion in modern America (though to be fair, Under God steers away from that particular emotion for the most part). Yet you will search in vain for something that will personally convict you or in any way challenge you to change your life. Of course, this is a history book--in that sense it should be all focused on the past. (Well, 'history' book.) But the stated agenda at the beginning of the book is to "ignite a passion and inspire you to learn more about the great heritage you have and to seek out the unfinished work left to do." (9) A noble enough goal, but not one met by Under God.
Really, the best thing I can do is point you to Miles Mullin's post on Thomas Kidd's blog, where he takes on the project of the group behind the publication of this book (the "Wallbuilders"). Mullin writes of how according to this worldview:
The United States has been uniquely blessed because of its Christian character...The same agenda is at work in Under God. This is idolatry of the second-highest order (the first-highest of course being actually worshiping statues), wherein the Christian life is identified with the life of the political community. Americans are not God's people--not even the Americans of the Founding generation. Only Christians can lay claim to that title, and even then not because of anything inherent to us (either where we're born or what nation we're born into), but only because of the grace of God and His kindness in Christ.
If the founders used Christian words, they must mean what we mean. Divorced from their context, quote after quote made founder after founder sound evangelical. Even Charles Carroll, the sole Roman Catholic signer of the Declaration of Independence, came across sounding like a good Baptist...
At the end of the presentation, [the listeners] were left with the following impressions: the founders were religious. They were religious just like me. Because they were religious like me, God has uniquely blessed America, per Psalm 33:12. And, if we want the blessings to continue, we need to elect righteous people.
As believers, we must remember that our country is ultimately not America--that is a temporary and passing thing. Our city has eternal foundations with God as its architect and builder. It will last forever, while America (and England, and France, and China, and Israel, and all the other nations of the earth) will eventually cease to exist, either through the providential working of history over time or immediately when Christ returns. There is no spiritual benefit that inherently comes from being an American--we may have different opportunities than Christians in other nations (easy access to good books, for example), but we are by no means better because we live in a nation that God loves more. As I pointed out in a previous post, America holds a place in God's providential plan for the world, but it is not any more special a place than that of any other country.
And, [sigh], I've got more of these books to read, so I should save something for later reviews. The short version is this: we must never put our faith in our nation, or in God's blessings on our nation. And we should certainly never assume that if only we all become Christians, America will become some sort of elect country. Our hope should be centered on Christ alone and His atoning work on the cross. Salvation does not come through having a country full of Christians, salvation comes to us when we reject our sin and embrace the Gospel offer in faith.
I've included some links below if you want to read more on the subjects mentioned in this post.
Previous Comments on Under God:
Bet You're Surprised I had this Many Thoughts
For Further (and better) Reading
Founding Brothers by Joseph Ellis
The Creation of the American Republic by Gordon Wood
Novus Ordo Seculorum by Forrest MacDonald
The Ideological Origins of the American Revolution by Bernard Bailyn
Freedom Just Around the Corner by Walter MacDougall
City of God by Augustine
Lectures on Calvinism by Kuyper
Natural Law and the Two Kingdoms and Living in God's Two Kingdoms by David Van Drunen
The Search for a Christian America by Noll, Hatch, and Marsden
Christianity and Classical Culture by Charles Norris Cochrane
Just a note: this last one is difficult and dense, but it's also a great introduction to how Christians interacted with the state in the first four centuries AD. I know of no secondary source that compares in terms of richness and depth for any other period of history...