The below passage confirming Douthat's take on American Christianity is from John Henry Newman's Essays, Historical and Critical (1839). According to Edward Short (who really likes Newman), "no better description of nominal Christianity has ever been penned."
In Newman and His Contemporaries, Short suggests that such remarks currently apply to American Catholics as well, "many [of whom] are indistinguishable from their erstwhile Episcopalian betters. They, too, preen themselves on their private judgment and cleave to a worldly, self-congratulatory, nominal faith."To tell the truth, we think one special enemy to which the American Church, as well as our own, at present lies open is the influence of a refined and covert Socinianism. Not that we fear any invasion of that heresy within her pale now, any more than fifty years ago, but it is difficult to be in the neighbourhood of icebergs without being chilled, and the United States is, morally speaking, just in the latitude of ice and snow. Here again, as our remarks will directly show, we mean nothing disrespectful towards our Transatlantic relatives. We allude, not to their national character, nor to their form of government, but to their employments, which in truth we share with them. A trading country is the habitat of Socinianism... Not to the poor, the forlorn, the dejected, the afflicted, can the Unitarian doctrine be alluring, but to those who are rich and have need of nothing, and know not that they are "miserable and blind and naked;"—to such men Unitarianism so-called is just fitted, suited to their need, fulfilling their anticipations of religion, counterpart to their inward temper and their modes of viewing things. Those who have nothing of this world to rely upon need a firm hold of the next, they need a deep religion; they are as if stripped of the body while here,—as if in the unseen state between death and judgment; and as they are even now in one sense what they then shall be, so they need to view God such as they then will view Him; they endure, or rather eagerly desire, the bare vision of Him stripped of disguise, as they are stripped of disguises too; they desire to know that He is eternal, since they feel that they are mortal.Such is the benefit of poverty; as to wealth, its providential corrective is the relative duties which it involves, as in the case of a landlord; but these do not fall upon the trader. He has rank without tangible responsibilities; he has made himself what he is, and becomes self-dependent; he has laboured hard or gone through anxieties, and indulgence is his reward. In many cases he has had little leisure for cultivation of mind, accordingly luxury and splendour will be his beau ideal of refinement. If he thinks of religion at all, he will not like from being a great man to become a little one; he bargains for some or other compensation to his self-importance, some little power of judging or managing, some small permission to have his own way. Commerce is free as air; it knows no distinctions; mutual intercourse is its medium of operation. Exclusiveness, separations, rules of life, observance of days, nice scruples of conscience, are odious to it. We are speaking of the general character of a trading community, not of individuals; and, so speaking, we shall hardly be contradicted. A religion which neither irritates their reason nor interferes with their comfort, will be all in all in such a society. Severity whether of creed or precept, high mysteries, corrective practices, subjection of whatever kind, whether to a doctrine or to a priest, will be offensive to them. They need nothing to fill the heart, to feed upon, or to live in; they despise enthusiasm, they abhor fanaticism, they persecute bigotry. They want only so much religion as will satisfy their natural perception of the propriety of being religious. Reason teaches them that utter disregard of their Maker is unbecoming, and they determine to be religious, not from love and fear, but from good sense.