Because the concept of the secular is normally conceived as standing in opposition to religion many people may not realize that it originally developed within a religious context. This may also come as quite a surprise to religious fundamentalists who decry the growth of secularism in the modern world. Rather than an atheistic conspiracy to undermine Christian civilization, secularism was originally developed within a Christian context and for the sake of preserving peace among Christians.
In fact, the concept that there is a difference between the spiritual and political realm can be found right in the Christian New Testament. Jesus himself is cited as advising listeners to render unto Caesar what is Caesar's and unto God what is God's. Later, the Christian theologian Augustine developed a more systematic division by distinguishing between two "cities," one that ordered the things of the earth (civitas terrenae) and one that was ordered by God (civitas dei).
Although Augustine used these concepts as a means to explain how God's purpose for humanity developed through history, it was employed by others for more radical ends. Some, who sought to reinforce the doctrine of papal primacy, emphasized the idea that the visible Christian Church was the actual manifestation of the civitas dei and, as a consequence, was owed greater loyalty than civil governments. Others sought to reinforce the principle of independent secular governments and used passages from Augustine which stressed the important role played by the civitas terrenae. This theological defense of autonomous civil powers would ultimately be the view that prevailed.
In medieval Europe, the Latin term saecularis was usually used to refer to "the present age," but in practice, it was also used to describe those members of the clergy who did not take monastic vows. These clergymen chose to work "in the world" with the people instead of removing themselves and living in seclusion with with monks.
Because of their working "in the world," they were not able to live up to the high standards of morality and personal conduct, thus preventing them from maintaining the absolute purity that would otherwise be expected of them. Those who did take monastic vows, however, were within reach of those high standards and as a consequence it wasn't unusual for them and for the Church hierarchy to look down a bit upon those saecularis clergymen.
Thus the separation between a pure religious order and a less-than-pure, this-worldly social order was very much a part of the Christian church even during its early centuries. This distinction was later fed as theologians differentiated between faith and knowledge, between revealed theology and natural theology.
Faith and revelation were long the traditional provinces of Church doctrine and teaching; over time, however, a number of theologians began to argue for the existence of a separate domain of knowledge characterized by human reason. In this manner they developed the idea of natural theology, according to which knowledge of God could be obtained not simply through revelation and faith but also through human reason while observing and thinking about Nature and the universe.
Early on, it was emphasized that these two spheres of knowledge actually constituted a united continuum, but this alliance did not last long. Eventually a number of theologians, most notably Duns Scotus and William of Ockham, argued that all doctrines of the Christian faith were fundamentally based upon revelation, and as such were necessarily filled with contradictions which would cause problems for human reason.
As a consequence, they adopted the position that human reason and religious faith were ultimately irreconcilable. Human reason must operate in and on the realm of empirical, material observation; it might arrive at the same conclusions as religious faith and the study of supernatural revelation, but they could not be united into a single system of study. Faith could not be used to inform reason and reason could not be used to structure faith.
The final push towards widespread secularization was not caused by anti-Christian secularists but by devoted Christians who were aghast at the devastation caused by the religious wars that swept across Europe in the wake of the Reformation. In Protestant countries there was initially an attempt to translate the principles of the religious community into the wider political community; that, however, failed due to the growing divisions between Christian sects.
As a result, people needed to find a common ground if they wanted to avoid civil war. This forced a reduction of overt and explicit references to specific Christian doctrines reliance upon Christianity, if it remained, became more general and more rationalized. In Catholic nations the process was slightly different, because members of the Church were expected to continue to adhere to Catholic dogma, but they were also allowed a degree of freedom in political affairs.
Over the long run, this meant that the Church came to be excluded more and more from political affairs as the people found that they appreciated having a realm of action and thought where they could be free from ecclesiastical authorities. This, in turn, led to an even greater separation between church and state than existed in Protestant lands.
The attempt to separate faith and reason as being different kinds of knowledge rather than different aspects of the same knowledge was not welcomed by Church leaders. On the other hand, those same leaders were becoming increasingly uneasy with the growth of rationalistic speculation in philosophy and theology. Instead of accepting the differentiation, however, they sought to repress that speculation in the hopes of holding on to the primacy of faith that had characterized Christianity for centuries while retaining rationalistic inquiry but on their own terms. It didn't work and, instead, moved outside the confines of the Church and into the growing secular sphere where people could work independently of religious dogmas.