This is what the entry on “Vegetarianism” in Joel B. Green’s Dictionary of Scripture and Ethics has to say:
The term vegetarianism was not coined until the nineteenth century, but as a practice, vegetarianism
was widespread in the ancient world. The Greek philosopher Pythagoras, who lived in the sixth century BCE, is thought to have been the first Western vegetarian, and esoteric groups associated with his name carried on this dietary practice well into the first several centuries of the Christian era. Several church fathers, including Tertullian and Clement of Alexandria, not only referred to the Pythagorean diet but also developed a Christian version of vegetarianism as an alternative to it.
Church fathers who defended a plant-based diet did so on several grounds. First, they were indebted
to Greek medical philosophy that attributed sexual and physical aggression to the consumption of
meat, especially the blood in meat. Second, they argued that Adam and Eve were vegetarians in the garden of Eden, since the animals were tame and there was no violence in paradise (Gen. 1:30). Third, they argued that God granted permission for humans to eat meat only after the flood, when
Noah and his descendants had to eat animals out of necessity (Gen. 9:1–4). Moreover, this permission was an accommodation to the fallen state of humankind. Fourth, they argued that the Jewish dietary law was evidence of a compromise between the peaceful diet of the garden of Eden and the gluttonous and cruel diet of the fall. Fifth, they argued that the council at Jerusalem, at which Paul and Peter divided their missionary responsibilities, kept in place the ban on consuming blood that was central to the Jewish diet (Acts 15:19–21). Sixth, and finally, they argued that the kingdom that God promised for the end times would include harmonious relations between people and animals, thus implying that vegetarianism was an appropriate way to anticipate the eschaton (Isa. 11:6–9).
Remarkably, all of these arguments were submerged in church history only to reappear in the nineteenth century, when many Christian groups rediscovered vegetarianism and began promoting
it for both theological and medical reasons. The modern case for vegetarianism is, thus, rooted in
the earliest Christian theologians.
Vegetarianism was never required as the daily diet for believers in the early church for a variety of reasons. First, it was associated with dualistic heretical groups such as gnostics and the Manichaeans. These groups identified the eating of meat with the consumption of fallen spirits embedded in animals, and they treated a plant-based diet as a work of merit necessary for the cleansing of the soul. They also were influenced by Eastern versions of reincarnation, which might have been one of the rationales behind Pythagoras’s version of vegetarianism.
Second, the early church was trying to distance itself from the more legalistic aspects of the Jewish tradition and, as a missionary movement, was not interested in erecting new barriers to fellowship. Third, many theologians argued that Jesus intended to free his followers from the more cumbersome aspects of Jewish legislation.
Nevertheless, vegetarianism remained as a sign of holiness, as demonstrated by two features of early Christianity. First, the church expected believers to fast on Fridays, a practice that slowly permitted the consumption of fish but no other animal flesh. The church also required a meatless diet for the period of Lent. Second, vegetarianism became the common diet of monasticism, with Benedict requiring it for monks in his rule, with the exception of the elderly and the ill.
Hobgood-Oster, L. Holy Dogs and Asses: Animals in the Christian Tradition. University of Illinois Press, 2008;
Linzey, A. Animal Theology. University of Illinois Press, 1995;
Webb, S. Good Eating.
Brazos, 2001; Young, R. Is God a Vegetarian? Christianity, Vegetarianism, and Animal
Rights. Open Court, 1999.
Stephen H. Webb (In Joel B. Green, ed., Dictionary of Scripture and Ethics, 2011)