The distinction between the concept of the “secular,” its derivative “secularization,” and the specific sociological theory of secularization is important; for the concept [of the “secular”] itself is so multidimensional that it is strangely reversible into its own contradictory meanings and, over the course of its history, has accumulated a wide range of different significations. It might even have been wiser to dispense with this concept altogether; but doing so creates even greater problems for sociology. The range of meanings of the concept of the secular and the contradictions among these meanings render it practically useless for the dominant modes of empirical-scientific analysis. Consequently, ahistorical positivist sociology must reduce this concept to clear, testable hypotheses so that it can be readily verified through longitudinal studies that attempt to take a census of the heads, hearts, and minds of religious people. But the complete elimination of the concept of secularization leads to a greater conceptual impoverishment; for in such a state we lose the complex historical memories that have accumulated in this concept and will lack the appropriate categories for charting and understanding this history. In the present secular society of America, an insular sociology of religion might be able to ignore this concept; but historical sociology cannot do so.

Let me recall just three meanings of this concept to illuminate the way in which these three meanings have become intertwined with the historical processes of secularization. By looking at the etymology of the concept of the secular, we learn that the medieval word “saeculum” had three undifferentiated layers of meaning. In the Romance languages there are equivalents of saeculum: Secolo, Siglo, Siècle.

These languages have preserved these three meanings. In Cassell’s Spanish Dictionary, opposite siglo we find: “century, age, world.” But in the contemporary secular “age” and “world,” of the three aforementioned meanings, only the meaning of “century” has retained its use in everyday life; for distinguishing time and space into two distinct realities, one sacred and the other profane, has, since times long past, truly become meaningless, even in Catholic Spain.

But in connection with Canon Law, a different meaning of secularization is derived; that is, where the term secularization refers to something that can be called an “act of law,” with legal consequences for the individual. Here secularization refers to the legal process by which a religious person leaves an enclosed place [in a monastery] and returns to the “world” and its temptations, thereby becoming a secular person.

In terms of canon law, priests could be both religious and secular. Those priests who decided to come out of the world of the saeculum and devote themselves to a life of the ideal constituted the “religious priests”; but those priests who lived in the world were the “secular priests.” Max Weber calls secularization the process through which the concept of the “calling,” that is, religious activities, migrates or is transposed into secular activities. That is, now for the first time, the concept of the “calling” denotes secular activities in the world. Here Max Weber uses the canonical concept of the “calling” as an analogy.

Finally, the concept of secularization refers to a real historical process. The term secularization originally meant the large-scale confiscation and seizure of church property and the expropriation of monasteries, endowments, and wealth bequeathed to the church by the state; this took place after the Protestant Reformation and in the wake of the continuing religious wars. Since then, secularization has come to mean the "passage," "transfer," and "displacement" of persons, things, functions, meanings, and so on, from their traditional place in the religious sphere to the secular sphere. Hence, it has become customary for secularization to denote confiscation—whether by force or due to failure to pay debts—as well as the appropriation of the functions of institutions that were traditionally in the hands of the church. This historical ambiguity, a remnant of the concept of secularization, only makes sense when we accept the fact that "once upon a time," a vast portion of reality in medieval Europe was indeed structured through a system of classification that divided the world into two heterogeneous spheres or domains: the religious sphere and the secular sphere.

The separation between these two spheres, in this specific and historically—relatively unusual—form, was akin to the sacred-profane division, which was certainly not the absolute heterogeneity that Durkheim always imagined it to be. For within the boundaries of this duality, there was much ambiguity, variability, overlap, and often manifest confusion, one example of which was the military orders. What is important to understand is that this duality was institutionalized throughout society. Hence, the social sphere itself had a dual structure.

There were "two swords," one spiritual and the other temporal, each claiming to have its own independent charismatic origin. A kind of institutionalized dual sovereignty that necessarily had to be the source of great tension and open conflict, and beyond that, efforts had to be made to end this duality by placing one of these spheres under the other.

The recurrent strife of the "office-holders" was the manifest expression of this ever-present tension. The theocratic claims of the church and spiritual rulers, who asserted superiority over earthly rulers and therefore held ultimate precedence and the right to govern temporal affairs, clashed with the caesaropapist claims of kings, who claimed to be the embodiment of sacred sovereignty by divine right [of kings], and with the efforts of temporal rulers who sought to make the spiritual sphere part of their patrimony and tributary.

A similar dual structure also divided "this world" into two separate spheres: the religious sphere and the secular sphere; these had to be distinguished from one another and kept separate from another division: "this world" and the "other world." To a large extent, the failure to separate these two types of distinctions has been a source of misunderstanding in discussions of secularization.

One might say that one should state more precisely that there were not "two worlds," but "three worlds." Spatially, there were the "other world" (heaven) and "this world" (earth). But "this world" itself was divided into the religious world [the church] and the secular world. Temporally, we find the same three divisions between God's eternal life and temporal-historical life, where temporal-historical life itself was divided into the sacred-spiritual time of salvation, marked by the church calendar, and secular life. From an ecclesiastical perspective, these three divisions were expressed in the eschatological distinction between the "invisible church" (the Communio Sanctorum), the "visible church" (the Una, Sancta, Catholica, Apostolica Roman church), and secular societies.

Politically, there were the transcendent City of God (the Kingdom of Heaven), its representative here on earth, the Church (the Papal Kingdom), and the City of Man (the Holy Roman Empire and all Christian monarchies). In the language of modern secular categories, we would say that there were two realities, one natural reality and the other supernatural reality. But supernatural reality itself was divided into non-empirical supernatural reality and its symbol, the sacred representation in empirical reality.

Thus it can be said that pre-modern Western European Christianity was constructed through two dualistic systems: on the one hand, the duality between "this world" and the "other world," and on the other hand, the duality within "this world"; that is, the duality between the religious realm and the secular realm. But the sacred nature of the Church meant that it mediated between these dualities. Because it was itself situated between these dualities, it belonged to both worlds simultaneously. It was therefore able to mediate between the two in a sanctified manner. Of course, such a classificatory system relied solely on the Church's claims that it could categorize reality according to its assertions; but only as long as people accepted these claims. In fact, the acceptance of the claims of the superiority of the religious realm over the secular realm—for whatever reason—could keep the inherent conflicts in such a dualistic system limited.

Secularization as a concept refers to a real historical process through which the dualistic system within "this world" and the sacred structures mediating between "this world" and the "other world" increasingly disintegrate, until the entire medieval system of such classifications completely disappears and new systems of spatial structuring of realms take its place. Max Weber's pictorial expression, "the walls of the monasteries have collapsed," is perhaps the best graphic depiction of this radical structural collapse; the wall that separated the religious realm from the secular realm in "this world" collapsed. But the separation between "this world" and the "other world" has remained, at least up to the present time. But from now on, there will only be "this world," that is, the secular world, and religion must find its place within this secular world. If previously it was the religious realm that seemed to encompass all of reality and within which the secular realm had to find its proper place, now it is the secular realm that encompasses all of reality, and the religious realm must adapt itself to it. But what place does religion have in these new systems of classification and differentiation that have emerged within this secular world?—if it has any place at all. This is precisely the analytical task of research on secularization theory.

Up to this point, our analysis of religion has been solely about the spatial structure, that is, about the place of the classificatory system that served to structure the social reality of medieval Christianity. We have said nothing about the individuals who lived in this social space, their religious beliefs, their religious practices, and their religious experiences. Perhaps one can speak with some certainty about the general dimensions of individual religiosity. Church membership was practically one hundred percent. With exceptions, some Jews and some Muslims were allowed to live in their own special quarters in Christian countries. Church membership was compulsory. But we know little about the religiosity of individuals. Everyone was Christian; even the dissenters and heretics—who were treated in the same inhumane manner that dissenters are treated in modern autocratic states. Dissent and heresy, which were regularly expressed, were understood either as a reform of Christianity or as a sectarian return to original purity, not as a rejection of Christianity. Regarding what is called the religious factor or the consequential dimension of religion, that is, the extent to which religion influenced [individuals'] behavior in the secular realm, it can be said that since life in the saeculum itself was, at least officially, regulated according to the principles proposed by Christianity, then by definition, Christians lived a Christian life within the bounds of Christendom.

Naturally, as in all societies, there were delinquent individuals within Christendom as well. In fact, the official view was that all persons were sinners: venial sinners, those guilty of mortal sin, those who lived perpetually in sin, and those who were excessively sinful and were excommunicated. Certainly, there were distinctions and tensions between [Christian] canon law, Roman law, and customary or Germanic law; but the distinction between religious sin, moral offense, and crime was not yet clear from a legal standpoint. In any case, we possess no reliable or generalizable information, not even scant information, about the statistical distribution of the various categories of sinners, or about the extent and intensity of religious beliefs, practices, and experience. Even when historians can determine with relative certainty the ratio of the number of priests to the number of religious individuals in society, these statistics in themselves provide little information about the actual religiosity of the society. However, we do have sufficient information about the widespread corruption in the papal court, as well as the unbridled indulgences in monasteries and among priests who engaged in the buying and selling of ecclesiastical benefices. If the [so-called] pious preachers lived in such a manner, then we have no reason to believe that ordinary Christians led more pious lives. In fact, the official Christian structure of society guaranteed that every individual lived according to [the standards of] the Christian religion. Because the religious structure itself was Christian, it was therefore no longer necessary for the lives of individuals—that is, the people living within this structure—to necessarily be religious.

Within this structure, there was ample room for both the intermingling and the separation between Christian authorities and the irreligious, as well as for public forms of religiosity. From the recorded documents concerning the struggles between orthodoxy and heresy, as well as the tensions between the religion of the church authorities and the religion of the laity, which ethnographers and social historians have unearthed, revisionist perspectives on religion in the Middle Ages and the early modern period are offered. We assume that the ideal-typical nature presented so far may be overly simplified; nevertheless, it is a fair characterization, and we can say with certainty that the hypothesis that pre-modern Europeans were more religious than modern Europeans is a hypothesis that precisely requires proof. Those accounts of secularization theory that begin precisely with such a baseless hypothesis and view the process of secularization as the process of the increasing decline of religious beliefs and practices in the modern world, in fact reproduce a myth that sees history as the progressive evolution of humanity moving from superstition to reason, from belief to unbelief, and from religion to science.

This mythical account of the secularization process, in fact, needs "desacralization." But this statement does not mean that we must discard secularization theory entirely; for what the sociology of religion needs to do is, instead of a mythical account of the secularization process in the world, to provide comparative sociological analyses of it and to ask whether these processes have occurred? And if they have occurred, when did they take place?

Secularization Theory

Once secularization found its place in social science debates, especially in sociology, where it finally found its home, any discussion of it, particularly any attempt to trace its genealogy and history, must begin in a highly paradoxical manner. Perhaps secularization theory is the only theory that managed to attain the true status of a paradigm within the social sciences. With the exception of Alexis de Tocqueville, Vilfredo Pareto, and William James, the secularization thesis was accepted in various ways by all the founding fathers of sociology: from Karl Marx to John Stuart Mill, from Auguste Comte to Herbert Spencer, from E. B. Tylor to James Frazer, from Ferdinand Tönnies to Georg Simmel, from Émile Durkheim to Max Weber, from Wilhelm Wundt to Sigmund Freud, from Lester Ward to William G. Sumner, from Robert Park to George H. Mead.

In fact, the consensus on the secularization theory did not even require testing; because everyone took it for granted. This situation means that although this theory, or better to say this thesis, often served as the unspoken foundation of many theories of the founding fathers of sociology, it was never precisely tested nor even explicitly formulated. The foundations for a systematic formulation of secularization theory are to be found in the works of Émile Durkheim and Max Weber. By freeing themselves from the positivist and Enlightenment critiques of religion—although Durkheim remained a professed positivist and Weber always considered himself a product of Enlightenment disenchantment—these two felt obliged to present, without illusion and with maximum scientific enlightenment, the foundations for the socio-scientific study of religion. By separating the question of the truth of religion from the question of its symbolic structure and its social functions, Durkheim's sociology provided a foundation for structural-functional analysis in anthropology and the sociology of religion. For his part, by abandoning the temptation to reduce religion to its essence and focusing on the task of studying the various meanings of religion as well as its socio-historical conditions and effects, Weber laid the comparative-historical and phenomenological sociological foundations of religion. Regarding Durkheim and likewise Max Weber, it can be said that the sociology of religion lies at the center of their sociological works. Although the theory of differentiation in the works of these two differs, it forms the core of their sociological theories, and the secularization thesis is the core of their theories of differentiation. Strictly speaking, secularization theory is nothing but a sub-theory of the general theories of differentiation of the evolutionary type, which Durkheim presented; but Weber, with differentiation of a more historical type, developed the theory of Western modernization. In fact, secularization theory is so intrinsically interwoven with theories of the modern world and with the self-understanding of modernity that no one can simply declare secularization theory invalid without calling into question the entire fabric of the social sciences, including a large part of their self-understanding.