Even if they are themselves believers, most people in the modern West find it all but impossible to reconstruct the religious mindset of ordinary people in earlier centuries. Yes, we are divided from them by views of the age and size of the universe, or the relative locations of Earth and Sun, but other points of division are still more basic.
Above all, it is difficult for us to realize just how thoroughly integrated religious ideas and terminology were into the cycles of the year, and the ways in which ordinary people defined the time in which they lived. In Europe, that meant a Christian cycle of time that is now all but unimaginable.
Even in advanced countries like England, right up to fairly modern times, Christians lived in a consecrated calendar. Rather, they lived that calendar. The shift from a peasant/agrarian society to an urban/industrial world created a chasm that was spiritual as much as economic, and contributed mightily to secularization.
In Western churches today, the Christian year has basically two main events, namely Christmas and Easter, and that is broadly true even for most adherents of liturgical communions such as the Catholic or Orthodox. In the Middle Ages, though -- from the fourth century through the sixteenth -- the calendar was much more comprehensive. The year was a recurring cycle of multiple commemorations, of days of feast and fast, all related to the key events of an agricultural world.
Each of these special days or seasons mandated particular kinds of behavior, in matters not just of food but of sexual expression. Even chaste marital sex was strictly prohibited during the (quite lengthy) times of fast. Weddings were permitted at certain seasons, but not others. The ritual year decided every aspect of the life, in the bedroom as much as the kitchen.
The central fact of religious life was Easter, but conceived much more broadly than today. The season of forty days before Good Friday marked the time of Lent, with the Easter season proper running from Easter to Pentecost or Whitsun. Lent and Easter combined thus made up ninety days, or a quarter of the whole year, with all the special regulations and prohibitions that implied.
Christmas again was a much more expansive season, beginning at the start of December, with Advent, and running right through to Epiphany on January 6. The time between Christmas and Epiphany was marked by a concentration of holy days and commemorations -- St. Stephen's Day, Holy Innocents (Childermas), St. John's Day.
The ritual division of the year changed at the Reformation, but not as much as we might think. Catholic and Orthodox countries, of course, retained the old cycles in their familiar form right up to the twentieth century, and those lands still constituted much of Christian Europe. But Protestant countries too still followed familiar patterns. In Protestant England, for instance, the local monastery might have been ruined, the pilgrimage shrine vandalized, and the parish church stripped of its "Popish" treasures and artwork. Even so, the year still followed old forms, enforced by highly intrusive ecclesiastical courts.
Without understanding these continuing cycles, it is all but impossible to understand much European literature written before the late nineteenth century -- certainly, many of the great novels, whether English, French or Russian.
Right up to that nineteenth century, economic life in the British Isles revolved around the Quarter Days, when all rents had to be paid, and by which laborers defined their employment contracts. England had four quarter days, namely March 25 (Lady Day, or the Annunciation, nine months before Christmas), June 24 (Midsummer, St. John the Baptist's Day), September 29 (Michaelmas, the Feast of St. Michael the Archangel) and Christmas.
Lady Day was especially important, commemorating as it did the Annunciation to Mary, and thus in a sense the beginning of the Christian story. Right up to 1752, this was the first day of the year, so that an English letter dated "February 10, 1725" was actually written in the year we would call 1726. In a Christian country, of course the year began with Christ. How could it be otherwise?
Four cross-quarter days also marked their own key events, and were particularly important in Scotland, long after the triumph of the Protestant Kirk. Most of these were also strictly ecclesiastical: February 2 (Candlemas, or the Presentation in the Temple), May 1 (May Day), August 1 (Lammas, "Loaf-Mass," the year's first harvest festival) and November 1 (All Hallows, the feast of All Saints). St. Martin's Day, Martinmas, fell on November 11 and had its special significance as the time when farmers slaughtered weaker animals in order to have food for the coming winter.
Each of these special days had its own distinctive roster of customs, folklore and superstitions, sometimes drawing on pre-Christian roots. Law courts and other national institutions faithfully followed the same markers. Cambridge University still has three terms, Lent, Easter and Michaelmas. Oxford has Michaelmas, Hilary (commemorating St. Hilary) and Trinity (Trinity Sunday is the first Sunday after Pentecost).
For a farming society then, religion marked time. If you asked someone the season, they would assuredly answer in terms that were religious, and distinctly Catholic and medieval: this is Lady Day or Whitsun, Lammas or Michaelmas. Now, it's a very open question what a given farmer could have told you about St. Martin, or indeed the theology of Easter, but they still lived the church's year. The faith was inescapable.
When we trace the story of secularization in the West, we can follow all sorts of markers and influences, including the rise of modern science and medicine. Undoubtedly, the population shift from country to town plays a massive role in the process. But this movement also fundamentally altered concepts of time-keeping and calendar observation, finally purging the old religious forms.
Urbanization and industrialization constituted a Reformation of Time.