Religion and Modernization: Sociologists and Historians Debate the Secularization Thesis (Oxford: Clarendon, 1992) ed. by Steve Bruce (photo above), is a collection of nine essays written by nine authors on the exact topic suggested by the volume’s title. After a short introduction by Bruce, the first full essay by Roy Wallis and Steve Bruce “outlin[es] the main elements” of “the ‘secularization’ thesis” (3) mentioned in the book’s title, which thesis is “one of sociology’s most enduring research programmes” (8). In the introduction, Bruce had mentioned that secularization is descriptive, rather than prescriptive, so that it
primarily refers to the beliefs of people. The core of what we mean when we talk about this society being more ‘secular’ than that is that the lives of fewer people in the former than in the latter are influenced by religious beliefs. […] (6)
If this phrase about “the beliefs of people” make it sound as though Wallis & Bruce are interested in individuals, the authors are quick to clarify that the theory they are advancing is really about “the diminishing social significance of religion”. This is not the same as “the decline of religion”, only of its “public role” (10). It is also certainly not an “even and irreversible decline” (27). Wallis & Bruce together reiterate that the “explanatory model” attached to this secularization thesis predicts that
the social significance of religion diminishes in response to the operation of three salient features of modernization
[…] (1) social differentiation, (2) societalization, and (3) rationalization. (8-9)
So that, in sum,
modernization […] brings in its wake (and may itself be accelerated by) ‘the diminution of the social significance of religion’. What features of modernization are involved? There seem to be three that are particularly salient: social differentiation, societalization and rationalization. (11)
Before examining these three features (social differentiation, &c.), we should ask: what is “religion”, such that secularization is the loss or diminution of the large-scale social influence of it?
If the role of religion in the secularization thesis makes the whole project seem to strangely dangle upon a definition of religion that isn’t on offer, Bruce quite oddly seems nearly to agree:
If […] we take the ‘bottom line’ of secularization to be changes in the religious beliefs and behaviour of individuals, we have to build our general explanations of secularization on a more detailed knowledge of religious belief and behaviour than we have at present. (6)
Bruce admits that, to obtain this definition in “times other than the present,” it would “require[…] the work of historians”; yet historians often use “unexamined generalizations” and draw on “the work of sociologists” (6) when describing things, so that sociology is thus the preferred discipline for answering the question: ‘what is religion?’ Sociology’s professedly empirical bent (3) is the reason for its preference: description should be beholden to quantification of some sort, to control for bias.
Before one can quantify something, one needs to know what feature of the world one is going to measure. What is religion, that one might measure it? Wallis & Bruce distinguish between functional definitions of religion, modularly identifying the work that religion does or might do, and substantive definitions, which define what it is (9).  Wallis & Bruce give four reasons for rejecting a functional definition (9-10). Nonetheless, a responsible case for a secularization thesis that is essentially related to religion still requires that religion be identified, defined.
Wallis & Bruce do have a definition of religion, albeit one that is strangely inclined towards belief in the reality of invisible (“supernatural“) agents, or even, in short, superstition:
that clutch of beliefs and actions predicated upon the assumption of the existence of supernatural beings or powers (9)
actions, beliefs, and institutions predicated upon the assumption of the existence of either supernatural entities with powers of agency, or impersonal powers or processes possessed of moral purpose, which have the capacity to set the conditions of, or to intervene in, human affairs. Further, the central claims to the operation of such entities or impersonal powers are either not susceptible to, or are systematically protected from, refutation. (10-11) 
It’s not clear to me whether this is insufficiently vague, or insufficiently specific: there is clarity here regarding invisible agents who “intervene”, but not regarding “powers” or “processes” that merely “set the conditions of […] human affairs”; is this definition lumping positions such as moral realism together with the practices and beliefs associated with leaving bowls of milk out for forest faeries? The appended qualification of religion as “not [being] susceptible to […] refutation” suggests that the answer is no, the authors are not lumping them together, but there is almost a rhetoric of unreflective popular practice rolled into the definition. In the end, Wallis & Bruce assure us that their definition is a “broad, contemporary, common-sense” one (10), and that it is useful for the task at hand (10). 
This definitional infelicity may simply be due to insufficient interdisciplinary habits within the university. Yet given what was said by Bruce in the introduction about “unexamined generalizations”, and the need for evidence-based theories, this manner of definition-creation seems rather peculiar. It would not be so peculiar if it didn’t ostensibly become the basis for what follows: “[o]n the basis of this definition [of religion] we can begin to spell out what is involved in the notion of secularization.” (11)  For an avowedly empirical venture to give the appearance of operating inductively or deductively from a “common sense” definition is strange. If it were not central to his definition of secularization, none of this would be worth mentioning.
The “three salient features of modernization” (8) which bring about the diminished public role of religion are  social differentiation,  societalization, and  rationalization. By social differentiation is simply meant
the process by which specialized roles and institutions are developed or arise to handle specific features or functions previously embodied in, or carried out by, one role or institution. (12)
So that the Church’s public role in education, health care, welfare and social control are outsourced to specialized institutions, which can train people in the specific bodies of knowledge peculiar to these roles: “religious officials will not be as highly trained [in these areas of expertise] as lay professionals.” (12) The same thing happens to the family’s roles of “production, education and social control” (12). This differentiation leads to an enormous variety of life situations, the end result of which is that
[t]he plausibility of a single moral universe in which all manner and conditions of persons have a place in some grand design is subverted. (12)
The variety of life situations can’t hold together; thus there is either fragmentation within a tradition so that each situation allows for a “metaphysical and salvational” system specific to it, or else there is fragmentation within the society of the religious tradition, so that one social class occupies that tradition, and other class or classes “openly oppose it”. (12)
The multiplication of life situations occurs in a social environment that is already much larger in scale than the village. The scale only continues to increase through  “industrial and commercial enterprise”,  “modern states co-ordinated through massive, impersonal bureaucracies” and  “the development of anonymous urban agglomerations as the typical residential setting.” (13) The locus of the individual’s life occurs on these larger impersonal scales, yet
The administrative concerns of industry and bureaucracy are rather different from those of chiefs within a community that itself has a single moral order. When scale and extreme variegation eliminates the “close-knit, integrated” communities necessary for religion’s public role, it “becomes privatized”, having less to do with role performance, and “more to [do with] privatized, individual experience.” (13)
The successful execution of administrative concerns within an impersonal scale requires technique, not virtue. Issues of scale thus go hand-in-hand with rationalization. Rationalization “involved the pursuit of technically efficient means of securing this-worldly ends” (14), most notably, technology. This is the desire for mastery unchained:
The growth of technical rationality gradually displaced supernatural influence and moral considerations from ever-wider areas of public life, replacing them by considerations of objective performance and practical expedience. (14)
With rationality and technology “reduc[ing] uncertainty” and chance, they displace “reliance upon faith” (14).
Social differentiation means a greater number of religions systems and options; the “greater the plurality of religious expressions available” means “the greater the variety of dissent” (15); more varied dissent means greater involvement in religious institutions in Protestant-dominated settings (16). There seems to be a relationship between [A1] involvement & [B1] variety on the one hand, and [A2] vitality & [B2] dissent on the other. The USA “represents the ‘universalisation of dissent'” (16). Variety is necessary for involvement-as-dissent: in Protestant settings, the greater the priority that the Anglican Church has, “the greater the decline in church involvement” (16). “Religious adherence […] as an expression of protest” (16-17) is potent.
Is protest fundamental? Is the desire for integrity, identity, and freedom the foundation upon which religious options are founded?
Cultural Defense and Cultural Transition
That dissent can be such a marker of mass involvement in a variety of religious institutions says something about the this-worldly work of these institutions. So Wallis & Bruce:
social differentiation, societalization, and rationalization generate secularization except where religion finds or retains work to do other than relating individuals to the supernatural. (17)
Such work fits into “two broad contexts, those of cultural defence and of cultural transition” (17).
With regard to cultural defense, “[r]eligion can provide resources for the defence of a national, local, ethnic, or status-group culture.” (17) This public role for religion can occur because groups on the periphery of a secularizing society are resisting secularization, or it can be because a national public is resisting the advance of an alien culture altogether (Poland and the Irish Republic were used as examples of the latter). “The more socially peripheral and culturally distinct the region, the more likely religion is to provide a focus of resistance, particularly when language no longer provides a viable basis for the assertion of cultural difference.” (18)
Religious institutions can also become increasingly public during times of cultural transition, such as during periods of large-scale immigration. The churches (or mosques, &c.) become sites for networking and integration. Sometimes people become more observant during and after this transition (though that trend doesn’t seem to continue for long). Therefore the authors state that
[a]lthough industrialization and urbanization tend, then, in the long term to undermine traditional community and thereby to subvert the basis on which religion can most readily flourish, in the short term they can be associated with an increase in attachment to religious bodies. (19)
Research shows that this increased attachment is short lived (19-20). Further, a “substantial modification of religious belief” (20) has occurred, in that religious adherents are increasingly less distinct from non-religious members of society, and “the sense of necessity appears to be disappearing from American religion” (20), polls showing that attendance at services was overwhelmingly thought to be non-necessary, and, among those who did attend, the motives shifted “from obedience to pleasure” (21). This does not mean that religion will disappear, but it does mean that it will not survive as a widespread social feature without artificial protections, because of the three features noted above:
Religion […] is likely to survive as privatized belief and practice, at society’s margins or in its interstices, and may even revive in times of trauma or major social transformation which may give it new work to do. Where it survives as a widespread practice it will tend to do so on the basis of an attenuation of what is specifically religious about it; that is at the expense of supernaturalism. Otherwise it will survive and be transmitted best in the private subjectivity of individuals and families, or in tight-knit sectarian groups which can cut themselves off from the world to varying degrees. It is in those settings where ‘societalization’ is held at bay by the creation of an insulated sub-society which preserves a distinctive sub-culture that ‘sectarian’ religion persists and grows. (21)
“Where [religion] survives as a widespread practice it will tend to do so on the basis of an attenuation of what is specifically religious about it; that is at the expense of supernaturalism.” Secularity, as worldliness, means the worlding of religion — which, for the authors, is not religion at all, but something else. 
Irreligious Pre-Modernity? Religious Modernity?
The two apparently common objections to this model are to assert that (A) the pre-modern world was never really religious as we suppose, that (B) in the modern world religion isn’t going away, or else (C) that we have substituted functional equivalents for religion that are, therefore, actually religious. In response to (A), they note that “lay participation was rarely encouraged and sometimes prohibited” in pre-modern religious forms (24), and that even “[o]utside the organized Church there was widespread supernaturalism” (25). In response to (B), the authors note that the “individualized, fragmented, and privatized” forms of religious belief and practice are exactly what are consistent with the loss of religion’s social significance (22), or else that, if it is public, that religion takes one of the two secular shapes (defense or transition) noted above. In response to (C), the authors note that the psychological needs that are similarly met when a culture moves from common, shared religious participation to common, shared sports participation “confirms rather than contradicts our analysis” and is, in truth, secularization (23).
Religion, one would think, is a either [A] a peculiar practice of a particular culture and epoch or cultures and epochs, making it the proper domain of historians, or else [B] a common feature of our humanity that is properly the domain of anthropology, not sociology. [A] is heavily sociological; [B] would always have sociological inflections, because common features of our humanity (such as language) all have local manifestations within and among a group of people.
Aside from this footnote, I will entirely ignore the odd counter-apologetic tone of the last sentence of the second citation above. Questions of authority (and assertions of revelation are certainly claims to authority) are undoubtedly an issue within the modern world in very forceful ways, but this was never properly explored in the essay, and so either the sentence should have been removed, together with similar sentences on (14), or else they should have said more.
I mention this because there are plenty of things we should call “religious” that are vulnerable to correction. What the authors wish to track is something that is not vulnerable to correction.
A substantive definition of religion, rather than a functional one, is here selected for reasons that are functional, rather than substantive — we have a definition of religion here that is not aimed at helping us understand religion (the authors almost admit that this definition is not up to that task), but secularization.
This definition doesn’t help their case. Surely, even without a definition of religion, we can see and feel that the transition to the modern world has brought with it massive shifts in social structure and collective identity; surely we can feel that the churches have lost their place as institutions for a nation’s corporate identity. If I might hazard a suggestion: we should be suspicious of any notion of the secular, secularity, or secularization that needs to define itself primarily in negative terms. I suggest it has a positive character of its own that is primary.
I disagree with their definition of religion, as it makes religion equivalent with something more or less like a belief in the agency of faeries. If that is what they wish to track and mark, so be it, but by calling it “religion” they turn a shared word with a huge semantic “common sense” range into one with a meaning proprietary to their project and subdiscipline. It is telling that, in five years of Masters degrees, I have not met anyone in graduate programs of religious studies who use the word “religion” in that narrow way — and many of these professors and students are not practicing, taking a more anthropological or philosophical approach to their craft.