The book of Leviticus defines animals that were ceremonially clean and therefore acceptable as food, as well as those that were ceremonially unclean and not to be eaten by the Israelites. An animal that had a split hoof that was completely divided and that also chewed the cud was considered clean (l,ev. 11.1-8.). A camel chews cud, but its hoof is not split; therefore, it is ceremonially unclean. But a sheep is clean, since it both chews cud and has a split hoof. A pig is unclean; although it has a split hoof, it does not cbew cud.

The pig became the symbol of something repulsive to the Israelites. It had been offered as a sacrifice in the pagan Canaanite worship, which had called forth Isaiah's harsh words of condemnation of those who ate "swine's flesh, and the abomination, and the mouse" (Isa. 66.17). A fair woman who turned aside from discretion was compared in the book of Proverbs to "a ring of gold in a swine's snout" (Prov. 11.22).

No one knows whether the herd of pigs that ran down the hillside into the lake was owned by Jews or by Gentiles. It is clear, however, that Jesus aroused the anger of the local people, who saw that a profitable business was being threatened, even though such profits were contrary to Jewish religious traditions and laws.