Hastings' Dictionary of the Bible (1898)
CLEAN, and UNCLEAN'. Lev 10:10. These words are applied in the Jewish law to persons, animals, and things. In order to partake of the privileges of the Jewish Church, the individual must not only be circumcised, but be ceremonially clean. How the various kinds of uncleanness were contracted, what time it continued, and what was the process of purification, are particularly described in Lev 11-15; Num 19. The division of animals into clean and unclean existed before the Flood, Gen 7:2, and was probably founded upon the practice of animal sacrifice.
Those animals only which divide the hoof and chew the cud were regarded as clean. Lev 11:3-4. The same chapter enumerates a variety of beasts, birds, reptiles, fishes, and things which are unclean. See also Ex 22:31; Ex 34:15, Acts 11:26; Deut 14:21. One object of these distinctions was to guard the Jews against heathen idolatry. Eating with the Gentiles was regarded as a peculiar aggravation of the offence of associating with them. Matt 9:11; Acts 11:3. Some of the prohibited animals were unwholesome.
The Hebrews were taught by these strict regulations to habitually regard their relation to God, and to abstain from everything that should offend his infinite holiness or involve the least appearance of pollution. The distinction between clean and unclean animals and nations was annulled by a special revelation to Peter. Acts 10:9-16.