My own preferred explanation - which is doubtless a small part of the pantomime - is theological rather than sociological: Christianity has thrived in the United States by adapting its theology to the habits and mores of the American people, in a way that religion in Europe hasn't managed to do. America is an Emersonian country, and its religious innovators have invented an Emersonian form of Christianity - which some might suggest isn't Christianity at all, of course - that's nicely tailored to the broader culture in which it swims. Call it gnosticism, or Moral Therapeutic Deism, or just plain Americanism - it means Elaine Pagels and Karen Armstrong for highbrow audiences and T.D. Jakes and Joyce Meyer for the masses, and it works.
Well, for my part, I'm partial to what most might dismiss as too meta-level a theory, namely, that there is something in the human spirit that craves a unity of experience, and under the conditions of the modern world, that unity has been achieved by adjusting religion to prevailing social conditions. But first, it might be worthwhile to remark upon what is meant when it is said that this American religion "works". It is to say, I think, that America is more religious than Europe because American religion works for Americans, given the sociological conditions of the nation; it better enables them to navigate, adapt to, and achieve psychological (this is an important qualifier) peace within the American social environment. It is not exactly question-begging, but rather a matter of the pragmatic connotations of that little word "works"; to state that something works is to orient it towards some mundane end or purpose. One would not argue, unless one were exceedingly vulgar, that Christianity "works" by getting a man salvation, for this would be to reduce and commodify the faith, to present it as some sort of transaction.
Seen in this light, the notion that American religion "works" seems less theological than sociological; at a minimum the sociology precedes the theology as the formative factor. Moreover, this pragmatic function of American religion would better explain why America has the religion it has, not necessarily why it is more religious than Europe: one might chalk up the differences in religiousity to differences in intellectual culture and political history, and explain the quality of American religion as a local adaptation to social conditions. Perhaps, however, there is no necessity of drawing such hard and fast distinctions. Consider the tenets of what Christian Smith describes as Moralistic Therapeutic Deism:
Moralistic Therapeutic Deism consists of beliefs like these: 1. "A god exists who created and ordered the world and watches over human life on earth." 2. "God wants people to be good, nice, and fair to each other, as taught in the Bible and by most world religions." 3. "The central goal of life is to be happy and to feel good about oneself." 4. "God does not need to be particularly involved in one's life except when God is needed to resolve a problem." 5. "Good people go to heaven when they die."
The resonances of this cluster of sentiments with earlier forms of Deism ought to be obvious; that these sentiments express the truncated nature of religious commitment within the civilization that was emerging into maturity at the time of the old Deists also ought to be obvious. The idea of God as a cosmic orderer or Divine Architect mirrors the idea of society as a self-correcting mechanism, as found in the writings of early political economists; the notion that morality involves fair dealings with one another, preeminently, expresses the presupposition of that sort of commercial society. The notion that happiness and self-esteem are the ends of mortal life is the lubricant of a type of society that demands the incessant pursuit of temporal satisfactions as an engine of prosperity. The idea that God exists to resolve problems expresses the systemic necessity of downplaying religious convictions and confessions as bases of social order, and stipulates the role of religion in society: as a bulwark and ratification of the economic and political order. Religion takes its orders from the social order. Finally, the notion that good people go to heaven when they die is merely a capstone of the ecumenical, utilitarian conception of religion; religion exists to serve the social and economic order, which is pluralistic precisely because it is already commercial and secular - and if one wishes to be a little cynical about it, promises man a life in the beyond if he refrains from rocking the boat.
America is the hypermodern, commercial society par excellence; American religion, to the extent that it does fit the paradigms of Moralistic Therapeutic Deism and personal growth, is fitted to inculcate the habits and attitudes conducive to success within, and the maintenance of, that society. To that extent, it is an epiphenomenal religion, a superstructure which legitimizes the social order. Why, then, in the end, is America still more religious than Europe? Could not Europe have a religiousity fitted to its own social conditions?
Two points seem apposite. First, the forms of Christianity instrumental in the development of American society and culture were those intimately bound up with the emergence of modernity, political, social, and economic. It is not to be marveled that what was once a byproduct of a religious tendency - and associated political movements - would one day assume a dominant role, and reshape the religion that midwifed it. In Europe, on the whole, religion either became identified with the most baleful effects of modernity and progress (as in England), or was regarded as an impediment to progress and enlightenment (as on most of the Continent). Religion, in consequence, has withered. Second, man is indeed a being who strives to attain a unity of experience; insofar as the socio-economic order is what actually unifies us in America, Americans have reshaped religion to conform to it. In Europe, insofar as religion either underwrote a social order experienced as oppressive and unjust, or was taken to impede progress towards Enlightenment, Europeans established that unity of experience by jettisoning faith. This, I propose, is why Americans are not only more religious than Europeans, but practice a certain type of religion.