Social Classes in Agrarian Societies(1)
(Click here for graphical representation)

The Roman Empire was marked by the kind of strict class stratification typical of imperial, agrarian (agriculture-based) societies. The two great divisions were between the Aristocratic (landowners) and Plebian classes. In vernacular conversation, Plebs sometimes are referred to as "peasants," but you will see below that this term refers only to one sector of the Plebian class. In the Roman period, citizenship and wealth are distinct phenomena not necessarily related to social class. However, social class, wealth, and citizenship status all played a role in determining which laws applied to a particular individual and how those laws should be enforced. For a brief discussion of the distinctions between Roman legal practices and structures as compared to contemporary American ones, see this page on "The Roman Legal System" (esp. the second and third paragraphs under "Development of Legal Codes").

I. The Aristocratic Class
Aristocrats comprise the upper class of Roman society. While the combined membership of the aristocratic class numbered perhaps 10% of the population of the Roman Empire, they controlled 90% of the wealth. However, aristocrats are distinguished from the lower classes not only by the amount of wealth they control but more importantly by their ownership of real property.
In the first-century Roman Empire, this comprised one individual: the Emperor. This one individual typically acquired at least 25% of the annual income of the Empire; sometimes that figure ranged as high as 50%. Octavian "Augustus" Caesar is the one Roman emperor named in the NT, in the Lukan birth narrative.
Governing Class
Approx. 1% of population; this class of persons received at least 25% of the annual national income. Members of the Herodian house and imperial legates like Quirinius are NT examples of the governing class.
Retainer & Priestly Classes
These two classes combined numbered perhaps 5–8% of the population of the Empire. The group included scribes, bureaucrats, generals, priests, and others united in service to the political elite. In Roman Judæa, the High Priest was appointed by the emperor or the emperor's agents, so members of the high-priestly families came to belong to the retainer class. Younger priests and those of less affluent means were members of the priestly class. Along with the unnamed Centurion, the High Priests Ananias and Caiaphas are NT examples of the retainer class. The Sadducee families belong in one or the other of these two classes. Some scholars would say that the Pharisees also belonged to the retainer class, but the evidence for that claim is sketchy.
Merchant Class
Perhaps 3–5% of the population, this class provided luxury items and other consumable goods for the governing, retainer, and priestly classes. Merchants did not make the goods, but bought them from the artisan class and then sold them for a profit. One NT example of the merchant class is Lydia of Thyatira.
II. The Plebian Class:
As in any agrarian society, the vast majority of the population of the Roman Empire belonged to the plebian class. Combined membership of these lower classes typically approximated 90%, while combined assets amounted to perhaps 10% of the wealth of the Empire. Plebs typically had no land of their own and made subsistence wages or less. The hand-to-mouth existence meant that members of the plebian class were highly susceptible to downward social mobility as a result of economic troubles. Debt slavery was a common phenomenon for this class. The vast majority of early disciples of Jesus, especially among the Pauline churches, seem to have been plebs.
Artisan Class
Perhaps 5% of the population, this group made the goods bought and sold by merchants to the members of the aristocratic class. Their trade is "portable," so artisans are able to travel without losing their means of self-support. However, most artisans made only a subsistence wage, so their travel typically was due to poor economic circumstances and other forces of social dislocation. NT examples of the artisan class include Prisca, Aquila, and Paul of Tarsus—if not before the beginning of his evangelistic period,then certainly afterwards (but cf. Jerome Neyrey on the portrayal of Paul in Acts).
Peasant Class
Farmers comprise the majority of the population in an agrarian society (50–75%). During the Roman Imperial Period, the average farmer may have been able to retain about 1/3 of the crop produced while paying 2/3 of it in rents and taxes. A bad harvest, however, had a disproportionate effect upon the amount of the crop retained by the peasant because the rents and taxes typically stayed the same, regardless of weather conditions. Fishers (like some of Jesus' disciples, including Peter, James, John, and perhaps Mary Magdalene) fall within the peasant class. While they do not farm the land, they "farm" the sea and other waterways. Peasants are distinguished from all of the other classes by the fact that their livelihood is tied to geography. They cannot leave the land/sea where they live without also leaving behind their means of self-support.
Unclean/Degraded Classes
Those Plebians whose origins or occupations separated them from the artisans and peasants fell into the "unclean" or degraded classes. Some occupations were indispensable but viewed as degrading or even defiling (e.g., ditch-digging, mining, prostitution), so those who carried out these occupations belonged to the degraded class. Circumstances of birth that might cause one to belong to the degraded class would include bastardy or exposure. This group comprised perhaps 5% of the population. Many of Jesus' disciples fell into this category, but their names remain unknown to us.
Expendable Class
The "expendable" class averaged 5–10% of population in normal times, but could grow as high as 15% in times of war, famine, or other forms of economic distress. The group included those forced to live by charity or by their wits (e.g., beggars and cripples; underemployed itinerant workers; petty criminals and outlaws). Unlike the degraded class, this group contributes nothing to the social and economic systems; rather, its existence is parasitic. Members of this class are those of whom Ebeneezer Scrooge says (in Charles Dickens, A Christmas Carol), "Let them die and decrease the surplus population." Jesus identified himself with the expendable class when he left his ancestral home to become an itinerant preacher.
1. This outline is adapted from the model of social stratification developed by Gerhard E. Lenski [from Power and Privilege: A Theory of Social Stratification (New York: McGraw-Hill, 1966), 215–90] as summarized by John Dominic Crossan in The Historical Jesus: The Life of a Mediterranean Jewish Peasant (San Francisco: HarperCollins, 1991), 45–46. To accommodate what seems to be more the case for first-century Imperal Roman society, I have reordered two pairs of the classes: I moved the Priestly class above the Merchants, and inverted the Peasant and Artisan classes.