Jan Bremmer is an impressive figure. (–but do not confuse him with the political scientist Ian Bremmer, whom you should follow on Twitter.) I was first acquainted with Jan’s work as a ravenous undergrad taking courses on the classics, when a professor suggested a book of his along with Walter Burkert’s standard tome. He has quite a large list of publications under his belt (you can browse the Amazon.com selection), having written some of the standard secondary texts on ancient Greek (pagan) religion, and branching out into early Christianity and myths and ritual worship in general.
About fifteen years ago Dr. Bremmer took to writing a remarkably concise set of notes, in the form of a narrative, titled “Secularization: Notes Toward a Genealogy“, published in a collection of essays titled Religion: Beyond a Concept edited by Hent de Vries. (The article may be visible in Google Books, and this is great, because the pages of the Academia.edu upload didn’t scan well near the spine.)
Bremmer wants to be humble and say that “genealogical investigations of terms are always problematic. Who can claim to have read everything?”  He claims he can’t, and hasn’t, but offers “a modest beginning”.
He begins with etymology. The words “secular”, together with “secularism”, “secularization”, and “secularize”, are heard often, and just as often, they have radically conflicting meanings. Bremmer notes that
[e]ventually, [all modern terms with the root secular] derive from the Latin saecularis, the adjective of the noun saeculum. […] Originally, the term denoted a long period of time […]. Only in Christian times did the meanings of saeculum and saecularis undergo important changes. Prominent Christian theologians, such as Tertullian and Augustine, developed the notion of saeculum as the world in which we live, a world that is characterized by sin and the rejection of God. This notion remained alive  during the whole time Latin played an important role in the world of scholars and clergy, and it is therefore not surprising that saecularis gave birth to the terms secular, secularism, and secularization. 
The language of secularization, in its origins, is tied to the Christian heritage of the West.
This is not apologetics, and shouldn’t surprise anyone. Before reading Bremmer’s article, I had both read and been told that the current widespread use of the word “secularization” originated in the wake of the Thirty Years’ War (A.D. 1618-1648), and referred to the transfer of Church properties to the secular princes (and the world of lay people in general) in and through the Peace of Westphalia that occurred in the wake of the Thirty Years’ War. Wikipedia, like Bremmer, notes that “secularization” is first used for the transition of an ordained priest from being a monastic priest to being “in the world”, vi&., to being a parish priest. I often wonder whether there are earlier uses.  Certainly the word “secularization” in the older and the early 19th century uses (if not our contemporary uses) presupposes the Christian distinction between the temporal and the eternal, the profane and the sacred, the secular and the spiritual.
The word saecularisatio “[a]s a noun […] originated in France”, appearing there in 1611, where it is qualified by ut dicunt — “as some people say”. The 1646 usage by a French delegate to the Münster negotiations used the word, and the terms saecularisatio and secularizare became standard legal phrases in German to talk about “the closure of monasteries or the liquidation of goods of the Catholic Church”.  It appears this sense of these words was stable for about the next two hundred years, at least until the Reichsdeputationshauptschluss of 1803.
Then shifts towards our modern use of the word begin to occur (though there is doubtless a build-up following the 1803 Reichsdeputationshauptschluss –which uses “secularization” in this older Westphalian sense– that’s chronologically prior to the following examples). In their correspondence that occurred in the early 1890s (published in 1923),
the philosophers Paul [Yorck] von Wartenburg and Wilhelm Dilthey occasionally use the verb säkularisiren to indicate development toward a world in which man leads life independent from the Church. 
This is not dissimilar to what we see in Max Weber, only slightly the junior to von Wartenburg and Dilthey. In his The Protestant Ethic and the Spirit of Capitalism (1905), the capitalist work ethic is described as a secularization of the Puritan Calvinist work ethic, and “secularization”, as he writes in his earlier 1904 essay “The Protestant Sects and the Spirit of Capitalism“, is that “characteristic process […] to which in modern times all phenomena that originated in religious conceptions succumb.”  Weber, like Dilthey & von Wartenburg, is using the word “secularization” not in the modern specialized sense, but in a sense that is simply a metaphorical extension of the earlier 17th century use of the term — though here something originally religious (the Puritan work ethic) in motive and aim transitions to become something employed for non-religious motives and ends (the Capitalist work ethic).
This raises the question of whether the culture and the distinctive goods of the modern world aren’t merely the camouflaged –or, as some have put it, stolen— property of Christendom, assuming they were merely the transference of habits, values, and beliefs from one culture to another, assuming that they were not grown autonomously — just as with Church property being questionably expropriated. This question leads to a crisis of legitimacy: if, on the one hand, one admits that the goods are, in fact, acquired by theft, then there is something illegitimate about the modern world, and we must find another way to be — so I am told that Karl Löwith argued; if, on the other hand, one argues that the modern world has its own logic and integrity, and that its distinctives are not specifically Christian in their origin, then we are not in need of another foundation — so goes (I am told) Hans Blumenberg’s argument. (I’ve long had these books on my shelf waiting for a read-through and review, as the latter is a direct response to the former.) I’d first heard many of these beats from the German Lutheran theologian Wolfhart Pannenberg. In his book Christianity in a Secularized World, Pannenberg offered a similar ancestry to the word and its use. Here is a long-ish excerpt from the beginning of Pannenberg’s book concerning the question of the origin of secularization, the concept of which
has its roots in the Christian distinction between the spiritual and the worldly. ‘Secularization’ originally had a legal meaning as a term for restoring a member of the clergy to worldly status or for the transfer of clerical goods into worldly possession. The latter process was regarded, at any rate by the Roman Catholic Church, as a strictly illegitimate act, in that it was illegal appropriation of church property. This was because of the seizure of church possessions by the worldly authorities in the Reformation period and later after the Reichsdeputationshauptschluss of 1803 (‘The Final Recess of the Imperial Delegates’, at which there was a major territorial reorganization in Germany associated with the necessity of ceding the right bank of the Rhine to France) even by Roman Catholic rulers. The other side could regard the same process as a sign of progress, as an abolition of unjustified privileges enjoyed by the church and a return of property withheld from Christian people to their own control or that of their political representatives.
As H. Lübbe [Hermann Lübbe] showed in 1965 [in his Säkularisierung], a similarly contradictory evaluation can be noted in the transfer of the concept of secularization to developments in cultural history. In his view this transferred terminology only arose after the middle of the nineteenth century. Here, too, on the one hand the slogan ‘secularization’ was used as a demand for the emancipation of  cultural life, for example science or even moral conceptions, from the tutelage of church and theology. On the other hand there was a lament over the increasing detachment of culture generally from the church and Christianity, though up to the end of the First World War this was raised more on the Roman Catholic than on the Protestant side. Protestantism tended more to see the secularization of culture as a necessary historical process which also freed religious life from fetters which had been imposed by the Christian habits of Christian states and especially by the political establishment of a particular church as a state church.
The term ‘secularization’ was not used as a descriptive, value-neutral category to describe cultural developments until our century. It was used to denote the process of ‘ridding the world of magic’ (H. Becker, 1932 ) which had in fact come about in the history of modern civilization above all as a result of modern science and technology. Max Weber spoke of ‘secularization’ in similarly value-neutral terms, for example in his investigation of the Calvinistic roots of modern capitalism. Here he particularly had in mind the transference to the economic sphere of the virtues of the work ethic which had developed from the Calvinistic belief in predestination in connection with an ascetic life-style. These virtues proved to be the key to economic success in a completely new way.
Richard Fester spoke of a ‘secularization’ of the historical consciousness and historical thought as early as 1908, at a congress of historians in Berlin, in terms of ‘historical thought freeing itself from the fetters of the biblical-theological worldview’ (Lübbe, [Säkularisierung,] 56). His comments were still bound up with the solemn feelings which went with emancipation. 
Pannenberg, however, rejects any claim to locate the transition between the late Medieval and the Modern periods in terms of the history of ideas. He notes that overwhelmingly people assumed, during and long after the Reformation, that “religious unity is the presupposition for social peace” , so that families should migrate to regions where their religion was normative, but that, in the Netherlands, it was first realized that imposing religious uniformity had the opposite effect of grounding social peace. Religious tolerance began in the Protestant Netherlands, and in England after the 1688 Glorious Revolution. People like Hugo Grotius sought to establish the peace of Christian society and between states not on a particular religious confession, but on “a natural religion common to all human beings.”  Therefore
the basic concepts of law, religion, morals, and politics were reformulated on the basis of the question of what is universally human, of the ‘nature’ of humankind. The mutually conflicting positions of belief among the Christian confessional parties were bracketed off, and in place of religion  based on traditional authority, that which is common to all human beings, human ‘nature’, became the basis of public order and social peace. This became the starting point for a secular culture in Europe. 
The elevation of the secular power (vi&., the sovereign, the state) to determine the legality and establishment of spiritual institutions (vi&., the Church, or which church), further led to the secularization of life and culture.  Because law and politics were now explicitly grounded in this-or-that idea of what it was to be human, were rooted anthropologically (rather than theologically and confessionally), human nature and human rights increasingly held the place that religion held in holding culture together.  While morality, in the wake of psychoanalysis, has been seen as an imposition of socialization on the self-fulfillment of individuals, human rights are not seen this way, but “have the status of a quasi-religious invulnerability”.  Because of this, the anthropology in circulation tends towards equality and democracy, even if democracies end up as markedly oligarchical and the democratic “has more the function of political religion than that of a description of political reality in the modern large-scale states”. This anthropologizing of religion was why “natural religion” was such a topic among deists and others in the early modern period, and why they saw Christianity as the refined form of it. The extension of the principle of toleration to non-Christians first sidelined Christianity’s equation with religion, but with the rise of atheism, the same principles of toleration that extended between Christian sects now extended to atheists and from atheists to Christians, so that the religion is now seen as “something which is in fact there, but in principle can be dispensed with.”  The break was not the Renaissance or the Reformation, but the aftermath of the Wars of Religion (book here).
Thus Pannenberg cites Dilthey in suggesting that
the ‘first and most powerful motive’ for the reorientation of thought on universal human nature which took place in the seventeenth century is to be found in the division of the church and the destructive  wars of faith. However many connections there may be with earlier ideas –with Stoic natural law, with the idea of the Renaissance and with the Reformation idea of Christian freedom– the shift towards the secular society arose out of the compulsion of need, not out of the ideas of Renaissance and Reformation, and certainly not out of a rebellion against the God of Christianity [as Blumenberg and Karl Barth had suggested, only the first with approval]. 
Back to Jan Bremmer. If the religious conflicts stemming from the Reformations and the political developments percolating through rise of the modern State generated the need for this word “secularization” in the wake of Westphalia, it was the Protestant tradition that began reflection on this latter use of the word, likely “stimulated through loss of Christian influence in Germany”, given how non-religious Berlin became. Interestingly, it was largely only in the twentieth century that the word became standard, though there were notable Catholics who would go on to contribute to this discussion. The various phenomena that are usually collected under “secularization” were looked at by various 19th-century figures, though not under that label (by the early 20th century this has changed in the English-speaking world: Bremer notes that the word “secularization” and the dynamics of secularization are on display in the 1929 set of studies titled Middletown by the Lynds).
Religious reflection about, for, and against secularism and secularization played an important role in the rise of the term to prominence and the development of the specialized vocabulary for it we now know. Bremmer cites the use of “secular civilization” in a 1928 missionary conference , and later the theological debates of the 1960’s , mostly a reaction to John Robinson’s 1963 Honest to God and Paul van Buren’s 1963 The Secular Meaning of the Gospel, and finally Harvey Cox’s 1965 The Secular City (which I really should review here eventually) — as well as the responses to these books, such as the fabulous E. L. Mascall’s 1966 The Secularization of Christianity and others. In France, apparently, the term “secularization” only “received general prominence”  through both these theological debates and the wake of the Second Vatican Council. The thinkers who popularized it “already saw the shadows of the decline of the Christian West”. 
Fascinatingly, after all that, the term “secularization”, as we know it to describe the processes of modernity and modernization, was only really stabilized in about 1963 with the work of Peter Berger and Thomas Luckmann, having transitioned “from a purely descriptive term into a paradigm”. 
Jan Bremmer, “Secularization: Notes Toward a Genealogy” in Religion: Beyond a Concept ed. Hent de Vries (New York: Fordham University Press, 2008), 432-433
I suspect there may be earlier uses of the word, given the tensions that were in the waters already in the 11th-century Investiture Controversy, which has anticipations even in the Merovingian and Frankish eras; there may even be precedents in the 4th-century debates about Roman emperors opening up Catholic church properties to be used by Arians, for all I know.
“The Protestant Sects and the Spirit of Capitalism” in From Max Weber: Essays in Sociology ed. H. H. Gerth & C. Wright Mills (New York: Routledge, 2009), 307
Is that Howard Becker? –the bibliography on Wikipedia doesn’t show an entry for 1932, and the reference must be in the book by Lübbe, above.
Wolfhart Pannenberg, “The Dispute Over Secularization and the Question of its Historical Causes” in Christianity in a Secularized World transl. John Bowden (New York: Crossroad Publishing, 1989), 3-4
Pannenberg, Christianity in a Secularized World, 13-14
Pannenberg, Christianity in a Secularized World, 18-19
Header image found here.