Of the Quantity of Drink, Chapter 40
"Every one hath his proper gift from God, one after this manner and another after that" (1 Cor 7:7). It is with some hesitation, therefore, that we determine the measure of nourishment for others. However, making allowance for the weakness of the infirm, we think one hemina [about 10 ounces] of wine a day is sufficient for each one. But to whom God granteth the endurance of abstinence, let them know that they will have their special reward. If the circumstances of the place, or the work, or the summer's heat should require more, let that depend on the judgment of the Superior, who must above all things see to it, that excess or drunkenness do not creep in.
Although we read that wine is not at all proper for monks, yet, because monks in our times cannot be persuaded of this, let us agree to this, at least, that we do not drink to satiety, but sparingly; because "wine maketh even wise men fall off" (Sir 19:2). But where the poverty of the place will not permit the aforesaid measure to be had, but much less, or none at all, let those who live there bless God and murmur not. This we charge above all things, that they live without murmuring.
Naturally, people entering a monastery bring with them habits and expectations that they formed while living in the world. And while some of the vows that monks traditionally take (not the three evangelical vows of the friars, i.e., poverty, chastity and obedience) are obvious such as obedience and stability, that of conversion of life is the furtherst reaching. Indeed, it is in some ways the heart of monastic life, because both obedience and stability can be seen to be a part of this conversion of life. In this sense, the monastic vows are the fulfillment of St. Paul's admonition in Romans chapter 12:
I BESEECH you therefore, brethren, by the mercy of God, that you present your bodies a living sacrifice, holy, pleasing unto God, your reasonable service. And be not conformed to this world; but be reformed in the newness of your mind, that you may prove what is the good, and the acceptable, and the perfect will of God.
The people of St. Benedict's time would expect to drink wine...it was often the only way of drinking uncontaminated drink, especially after the disapperance of the Roman administration following the collapse of the Empire. Nevertheless, St. Benedict knew that it was all too easy to take too much wine, and that this would inhibit the life of Christ and the community for the monks. So this small amount (two glasses really) of wine per day is the allotment for well-off monasteries. Poorer ones are to go without, but most importantly, without grumbling.
This is of course, the key idea in St. Benedict's rule: that we are to be grateful for the grace of God, not grumbling about what we don't have. That grumbling may be just as much a part of human life as making mistakes, but we are supposed to learn from the example of the Israelites who left Egypt, but grumbled about the onions and garlic they left behind. Unfortunately, we all too often imitate those ancient children of Israel too closely.