When did we stop diluting wine?

I don't know the exact answer, but have the following conjecture.

In the old times water from many sources was not safe, and people did not know that one has to boil it to make safe. So they mostly drank wine (or beer, or vinegar diluted by water, as Roman soldiers did). Simply because there was no safe non-alcoholic beverages. If you drink only wine, you quickly became drunk. So wine has to be diluted. This also explains why they still drink diluted wine in some places in Europe. Because by tradition, in many places in Europe, wine is the main beverage. This tradition is preserved in some places in Europe, though of course nowadays many non-alcoholic beverages are available.

Once you have plenty of non-alcoholic beverages, or abundant clean water, you drink wine only for entertainment, so you can drink undiluted wine (is relatively small quantities).

Remark. Perhaps the comment of @congusbongus 5 is correct. But the fact remains that in many cultures people drank wine or beer as their main beverage. My friend came to small a Swiss village for vacation and rented a room with a local peasant. He asked the peasant: "Is water here safe to drink"? The answer was: "I don't know. I've never drank water in my life!"

Ancient Macedonians were known to drink wine undiluted with water ― a trait which their southern neighbors in Greek city-states like Athens considered barbaric. Other groups that liked to drink their wine straight included the Celts, Iberians, Thracians, Scythians and Persians.

However, what some cultures call barbaric is for other cultures the epitome of excellence. Consider the ancient Israelites. Following Nehemiah's wine expertise as a ruler in the Persian empire, wine was crafted to not be diluted. For example, Isaiah the prophet calls out to his corrupt culture, “Your silver is full of impurities and your wine is diluted with water."

However, in Talmudic times concurrent with Greek and Roman times, wine was frequently crafted to be diluted with water before drinking. And in early Christian tradition sacramental wine was frequently diluted as well, as Justin Martyr in chapter 65 of his First Apology implies. However, this was not a universal tradition. The Armenian Apostolic Church, which is one of the most ancient branches of Christianity, has from its beginning used only pure (not diluted) wine for communion.

The second century writer, Clement of Alexandria argues:

It is best for the wine to be mixed with as much water as possible. . . . For both are works of God, and the mixing of the two, both of water and wine produces health, because life is composed of a necessary element and a useful element. To the necessary element, the water, which is in the greatest quantity, there is to be mixed in some of the useful element.

Isaiah Cox in his "Wine Strength and Dilution" appeals to a Jewish understanding of wine dilution, articulated in the 11th century, and points out that (emphasis added):

There is a common understanding among rabbonim that wines in the time of the Gemara were stronger than they are today. This is inferred because we know from the Gemara that wine was customarily diluted by at least three-to-one, and as much as six-to-one, without compromising its essence as kosher wine, suitable for hagafen. While repeated by numerous sources and rabbonim, the earliest suggestion that wines were stronger appears to be Rashi himself.

What most likely happened was that after the fall of Rome, the center of the wine industry shifted to the more northern areas of France & Germany. Because those in the north both crafted and preferred their wines to be served undiluted, that became the new bench mark style.

It is likely that in Greek & Roman times, water was frequently added because the wine was “stronger” than our modern wines in the sense that the after-effects were far more potent, meriting dilution. Typical reasons for why this would be the case might be due to the following:

1. Late harvest picking of ultra-ripe grapes (some branches cut off to allow raisins to develop), resulting in a massively high alcohol or at least sickly sweet wine:

In the eighth century BC, Hesiod writes of picking the grapes in bunches, “and bring your harvest home. Expose them to the sun ten days and nights, then shadow them for five, and on the sixth, pour into jars glad Dionysus’s gift.”...Cato recommended drying grapes for two to three days, while on the island of Thasos, they were dried in the sun for five days, and on the sixth were plunged into a mixture of boiled grape juice and salt water. See here.

2. Tannic astringency taking place through too much stem inclusion &/or accidental inclusion of unripe grapes in the fermentation process
3. Boiling of grape must prior to fermentation. This article points out:

Cato, Columella, Pliny, and Palladius (On Agriculture, XI.18) all describe how unfermented grape juice (mustum, must) was boiled to concentrate its sugar. "A product of art, not of nature," says Pliny (XIV.80), the must was reduced to one half (defrutum) or even one third its volume (sapa), and the thickened syrup used to sweeten and preserve wine and fruit that otherwise was sour or would spoil.

4. Adding of honey, grape syrup (σιραίος) or other sweet fruit during the fermentation process, resulting in a higher alcohol wine. Note: Some ancient yeast strains may have been very alcohol-tolerant. In describing Falernian wine, Pliny the Elder appears to have alluded to this as he (comically?) noted, "It is the only wine that takes light when a flame is applied to it." Although, it is also possible that Pliny's reference to a case of Falernian wine catching fire was due to a type of tree resin floating on its surface.

5. Microbial contamination of wine (Brettanomyces, Zygosaccharomyces, etc.) that is more apparent in a sensory threshold sense when not diluted

6. Evaporation of water taking place in the skin/barrel aging process (i.e. like a reduction sauce)

7. Resin leakage in storage vessels

8. Defrutum was sometimes used to sweeten or preserve potentially sour wine (Pliny, XIV.121).

9. Honey was sometimes added after fermentation. One contemporary writer describes a wine called mulsum as:

... a white wine sweetened with honey that often was freely dispensed to the plebs at public events to solicit their political support. It was not necessarily inexpensive or inferior, however. Martial writes that the best quality was made of Falernian mixed with Attic honey, a drink suitable to be poured by Ganymede, himself, cupbearer to Zeus (Epigrams, XIII.108). And Pliny agrees that "the best honey wine is always made with old wine" (XXII.liii.113). Varro relates the story of Appius Claudius Pulcher, brother of the notorious Clodia (the "Lesbia" in Catulllus' poetry) who in his youth served muslum to his guests but supposedly was too impoverished to drink it himself (De res rusticae, III.16.2).

10. Some wines were deliberately made with extra additives for medicinal purposes or to facilitate psychedelic experiences. For example, outside Pompeii, an ancient pharmacy was unearthed. Inside wine jars was a mixture that included lizard bones, opium, cannabis, and henbane - an hallucinogenic Solanaceous plant.

Shaun Anthony Mudd in his book, Constructive Drinking in the Roman Empire: The First to Third Centuries AD writes the following:

Dioscorides also advised specific flavoured wines for similar purposes: those flavoured with resin for aiding digestion, with germander for treating slow digestion, and with goat’s marjoram or Cretan thyme or savory or oregano for combating indigestion. Pliny similarly considered wine, must, or raisin wine seasoned with resin be a useful therapeutic measure for treating overly cold stomachs, and the wine called bion (from the Greek for ‘life’; made from sun- drying unripe grapes) to be extremely useful for treating many illnesses, including disordered stomachs, weak digestion, intestinal problems.

Concerning the alcoholic, or perhaps just the astringent strength of wine, it was sometimes said in those days:

In daily intercourse, to those who drink it moderately, it gives good cheer; but if you overstep the bounds, it brings violence. Mix it half and half, and you get madness; unmixed, bodily collapse (Athenaeus quoting Mnesitheus of Athens in Deipnosophists 2. 36a,b).

Pliny laments that "genuine, unadulterated wine is not to be had now, not even by the nobility" (XXIII.1). He writes: "So low has our commercial honesty sank that only the names of vintages are sold, the wines being adulterated as soon as they are poured into the vats. Accordingly, strange though it may seem, the more common the wine is today, the freer it is from impurities" (XXIII.34).

Still, there were exceptionally good wines that were enjoyed without being diluted. One writer notes (emphasis added):

One of the great wines of Rome – it is mentioned in many of the most prominent texts and poems – was Falernum, which came from Campania, near the border with Latium. There are many references to the exquisite quality of the wine and especially to the spectacular vintage of 121 BC, which was known as Opimian, after Opimius, who was Consul in that year. Opimian wine was clearly a byword for connoisseurs – a Roman Robert Parker would have given it VC, if not C points out of C. In his Satyricon, Petronius has his banquet host bring out bottles labelled, “Falernian. Consul Opimius. One hundred years old.”

So good was Falernian wine that writers proposed drinking it straight, rather than diluted with water or must, or flavored with herbs and spices.

Even if Ancient Greek & Roman wines were made in an excellent manner, without it being too "hot", it might have been just a matter of wine palette preferences or a matter of habit that led to the desire for wine dilution to take place. For example, one can take a good bottle of modern Pinot Noir, add it to water and come up with a nice fruity water to drink. A similar analogy might be found in coffee consumption. For example, many people who drink copious amounts of coffee make it with more water than the connoisseurs who drink just a couple of cups a day.

The above being said, undiluted wine was commonly given to those suffering from the chills (Celsus, On Medicine, I.3.10). Old men, too, were encouraged to drink undiluted wine so as to warm themselves (I.3.32). See also Aristotle, On Rhetoric, II.13.

In short, in many cultures, wine crafted with excellence is made to be appreciated without being watered down. And that is something that the Romans & Greeks did not grasp, like the Persians and other cultures did. It took the fall of Rome for a shift in wine practices to take place.