What would happen if I, a peasant, didn't show up to Church in Catholic Europe in the Middle Ages?

Eh, you'd be missing out on the most active cycle of the local gossip mill, skip a chance to hang out with your friends, risk missing a major event (like a celebrity preacher's sermon after Mass) that people would be talking about for awhile.

Or, you know, you might BURN IN HELL FOREVER!!!11!1!

A useful way to think about the history of the western medieval and early modern Church is as a history of getting more and more people to church. Our picture of lay religious life in the early Middle Ages is unfortunately hazy. The basic emphasis, though, was on baptism--which in practical terms, meant the basic emphasis was on getting adults to bring their infants for baptism. That was the crux (...) of being a Christian. Parents would have to attest they understood basic elements of Christianity (recite the Our Father, the central prayer of the medieval Church; know the Creed, a poem-like assertion of Christian dogma). But weekly church attendance? Honestly, scholars have debated whether early medieval priests even preached sermons at all. (Bishops did, but the average priest?)

From around the year 1000 right through the Reformation and Catholic Reformation, a series of escalating reform movements swept through the western Church. I say "escalating" because each movement was driven by/involved/affected increasingly large and lay groups of people. It started off aristocratically enough, with a few nobles pushing to reform a few monasteries for more effective prayers for the noble families' souls. In the twelfth century, along with the major intellectual developments both inside and outside of religion, we see lay adults take a interest in remaking their entire lives in a "Christ-like" manner. This might mean going on crusade; this might mean running off to the forest to become a religious hermit; this might mean helping found one of the new, more rigorous monastic orders; this might mean the Church in Rome thinks your town's religious revival is dangerous and anti-establishment (read: anti-them) and wants to burn you all at the stake.

Spurred by the activity, excitement, and paranoia over heresy that burbled up in the 12C, the Fourth Lateran Council of 1215 made one of the most important and famous declarations in Church history, known as Omnis utriusque sexus:

All [Christians] of both sexes shall, after they have reached the age of discretion, faithfully confess all their sins at least once a year to their [parish] priest, and perform to the best of their ability the penance imposed, receiving reverently at least at Easter the sacrament of the Eucharist.

Omnis utriusque sexus, thus, added 2 church visits every year to the requirements for a Christian. (1) come to confession during Lent so you would be in a state of grace for (2) reception of the Eucharist--the Body of Christ; no wine/Blood for the laity--on Easter. It's not an accident, by the way, that the 12th-13th century also codifies reception of the sacraments, and specifically these, as pretty much the most important activities for Christians on earth (baptism, which remits original sin, is the sine qua non, of course).

From 1215 into the early fifteenth century, we can say a couple of things about Church attendance and requirements. First, it obviously took Canon 21 some time to be successfully implemented. It's really hard to sketch out the speed with which it spread geographically and in terms of enforcement and excitement, which probably doesn't come as a surprise. However, scholars agree that by 1400 or so, lay Christian in western Europe were indeed going to confession around Palm Sunday or so and receiving the Eucharist (bread only!) on Easter as a general practice.

Second, also by 1400, this was probably not the only time most lay people were in church! For one thing, although reception of the Eucharist was a very limited occurrence outside the clergy, monasteries, and some communities of overly devout semi-lay women, the public performance of the sacrament--in terms of consecration of the elements and consumption by priests--was absolutely not. A sort of secondary visual reception of the sacrament was considered to have benefits for the soul, too--in fact, to be safer in a sense, because watching someone else receive the Eucharist was an act of prayer and devotion on your part, but you didn't risk hellfire for receiving it when you hadn't been absolved of all your sins. WIth incense swirling around the altar, bells ringing, elaborate staging--even watching the consecration and priest's reception was a central, and popular, moment for late medieval laity to commune with the sacred.

For another, over the course of the later Middle Ages, the sermon became a centerpiece of Christian ritual life. Sometimes preached during Mass, more often afterwards (and sometimes not even attached to a church service at all), sermons became one of the emotional and devotional touchstones of Church life for medieval laity. Rural villages in 15C Germay find their one peasant who can write a few words to complain to the local bishop that they need a better priest, one who can actually preach. Booklets of sermon collections are one of the hottest-selling genres of early print. It's not an accident that sermons and preaching become the key to the tsunami of the early Reformation, both as a vehicle of its spread and as a theological/pastoral issue. By the fifteenth century and increasingly over its course, sermons mattered.

So this gives a sense of what Church attendance looked like "generally". The requirement to present your kids for baptism, the requirement to confess your sins during Lent, the requirement to receive the Eucharist (in one kind) at Easter. Plus a fluctuating array of additional options (including, by 15C, some jurisdictions offering confession/the Eucharist to the laity up to four, count 'em FOUR, times a year!). Was there enforcement?

We can point to a few things, absolutely. Canon 21 already prescribes excommunication for people who do not show up for confession and the Eucharist. In practical terms, this meant...well, it meant you couldn't receive the sacraments the next year. :P Nah, the real implications were soteriological (related to salvation): you were going to die out of a state of grace, go to hell, and you couldn't be buried in consecrated ground. There was also public penance to be done when excommunication was lifted, in addition to private penances of prayers, pilgrimages, and so forth.

In 15th century handbooks for confession, that offered some form of guidance to priests on how to perform that sacrament (scholars still debate exactly what form of guidance...the genre is troublesome), you can start to see occasional references to weekly (Sunday) Mass attendance being a norm or a rule. However, it's not a mortal sin to skip watching the Eucharist like it is to miss receiving it. You can even find some that talk about nuns skipping Mass specifically to do needlework being the problem!

Actually, that last part gets picked up by some civic ordinances, which require guilds to seize the profits of their members that were gained by working on Sundays. That's not quite a reference to missing church, but it falls into the general realm of keeping the Sabbath. (And besides, the rest of "keeping the Sabbath" generally involved food and hanging out and reading gory saints' lives to your kids if you were literate and owned a book. If you were an urban artisan in the 15C you might well have, so.)

One additional set of consequences is ephemeral, but crucial: social. There would be the rumor mill as to why a person needed to skip confession, of course. But more to the point, Church life--the sacraments, celebrations afterwards, participation in a local confraternity or religious guild, upkeep for the local church building--was the center of late medieval culture, even for the laity. (Even craft guilds had heavy religious involvement as a collective). Surely most people skipped a Mass from time to time, ducked out of a sermon (although why?--our general sense is that people's attention was...not always on the preacher, shall we say), and so forth. But repeated refusal, and of course skipping reception of the sacraments, would cut a person out of the lifeblood of their community.