Animals and fish designated ‘unclean’ were forbidden to be eaten; the ‘clean’ were permitted. The distinctions are clearly written into the Pentateuch (e.g. in the time of Noah, Gen. 7: 2). This suggests that as elaborated after the Exile by the P source in the tradition, they may have coloured the narratives of earlier periods. Roughly, birds and animals that devoured their victims were unclean, because they consumed blood. A full list of unclean creatures is given in Lev. 11. People might become unclean—for instance, if they touched a dead body (Num. 19: 11 ff.; cf. Luke 10: 31) or came into contact with leprosy (Lev. 13: 3) or bodily fluids (Lev. 12: 2). Means of ceremonial purification from the effects of defilement were prescribed.
The origin of these distinctions may have been hygienic; for example, the experience of food poisoning. But the effect of the legislation was to emphasize the difference of Israel as a holy people separate from other nations. It was precisely this notion of separateness that Peter was reluctantly persuaded to renounce (Acts 10: 15) as being inconsistent with Christian fellowship. Much of Paul's energy was given to teaching that for Christians the time of the Law, including its food regulations, was over (Rom. 14: 15), and Jews who joined the Church should renounce all their ties with the synagogue and throw in their lot with those converted from paganism. There should be one community in faith and worship and fellowship (Rom. 15: 7) on Pauline principles.