After the fall of the Western Roman Empire, feudalism and Christianity take hold in Germany and most of central Europe. There are hundreds of recorded laws in the first millenium C.E. that specifically address beer production. The most interesting (at least to me) is that even the great Charlemagne had laws written regarding beer production. He was even known to drink and enjoy beer himself, but usually not more than three cups with a meal (source). It was under the rule of Charlemagne that we begin to see the massive expansion of the church and by extension, monasteries.
In early Europe, a monastery was the center of life for many towns and villages. Just like your average citizens, monks also brewed but the end result was still not beer as we know it today. The early Middle Age brewers made a similar product called gruit. Gruit is a malt-based beverage flavored with herbs. It is impossible to be more specific than that as brewers of the day used ALL available herbs, some of were even narcotic in nature. There is evidence of this practice from the British Isles to France to Scandinavia. In fact, Finnish brewers still make a beer-like beverage called sahti which is spiced with juniper.
The biggest difference between gruit and beer is hops.
According to the German Beer Institute, hops have been farmed dating back to roughly 730 C.E. Hop usage seems to have arrived during the Carolingian Era as the first recorded mention of hops in beer was written by St. Adalhard of Corbie in 822. Research conducted by beer historian Richard Unger suggests that hops were regularly grown in monastic gardens as early as the 11th century. Hops farms from the Middle Age have been discovered all over Europe. By the early 1300s, it seems that the use of hops in beer was rampant (source).
Hops became widespread for multiple reasons but the most significant was preservation. The oils found naturally in hops allowed beer to be stored for months under the right conditions. There were not many foods that could last that long during this time. Now that beer could last longer and was cheaper to make (less spoilage reduces costs dramatically) it really took off.
Beer was so popular that by the late 1400s Germany was facing a shortage of grains, specifically wheat and rye. Bakers did not have enough of these grains to make bread and so the Reinheitsgebot was enacted. First put forward in 1487 but not passed until 1516, the Reinheitsgebot, better known as the German Purity Law, limited brewers to three ingredients: water, barley, and hops. Beer was still fermented in open air vats at the time so yeast was unknown to brewers until Louis Pasteur discovered the role played by microorganisms in fermentation in 1857. Yeast was then added to the Reinheitsgebot. The Reinheitsgebot is the oldest and still valid food quality law in Germany (German Beer Institute).
From the Reinheitsgebot we can jump to beer in America. There are breweries today that have been in existence since before the Purity Law was enacted, the oldest being Weihenstephan in Bavaria which has been operational since 1040 and maybe even before that! For our purposes though, we are now up to the point where we would recognize the product being made as beer even today.
If you are interested in any of the styles mentioned above the American craft brewers Sam Adams, New Belgium, and Dogfish Head have all produced a variation of Sahti in recent years. Austin, TX brewers, Jester King, produce Gotlandsdricka, a beer flavored with sweet gale, juniper berries, and smoked malts reminiscent of the beers made by Vikings (I’m an unabashed JK fanboy and my friends have this waiting for me when I return, yum!).