Boyd’s Bible Dictionary:

The hog was pronounced unclean

Concise Bible Dictionary:

One of the animals classed among the unclean, and which is supposed to have been held in abhorrence as food by the Jews. The prophet Isaiah, however, charges them with eating swine’s flesh; and their apostasy was such that he says when they offered an oblation, it was as if they had offered swine’s blood: their heartless profession was abhorrent to God (Isa. 65:44Which remain among the graves, and lodge in the monuments, which eat swine's flesh, and broth of abominable things is in their vessels; (Isaiah 65:4); Isa. 66:3,173He that killeth an ox is as if he slew a man; he that sacrificeth a lamb, as if he cut off a dog's neck; he that offereth an oblation, as if he offered swine's blood; he that burneth incense, as if he blessed an idol. Yea, they have chosen their own ways, and their soul delighteth in their abominations. (Isaiah 66:3)
17They that sanctify themselves, and purify themselves in the gardens behind one tree in the midst, eating swine's flesh, and the abomination, and the mouse, shall be consumed together, saith the Lord. (Isaiah 66:17)
). It is not recorded whether the Gadarenes were Jews or Gentiles, who lost their swine by the demons’ possession of them (Matt. 8:3232And he said unto them, Go. And when they were come out, they went into the herd of swine: and, behold, the whole herd of swine ran violently down a steep place into the sea, and perished in the waters. (Matthew 8:32); Mark 5:1313And forthwith Jesus gave them leave. And the unclean spirits went out, and entered into the swine: and the herd ran violently down a steep place into the sea, (they were about two thousand;) and were choked in the sea. (Mark 5:13)). The swine are typical of the most defiled and degraded, to whom apostates are compared, and before whom holy things should not be cast (Matt. 7:66Give not that which is holy unto the dogs, neither cast ye your pearls before swine, lest they trample them under their feet, and turn again and rend you. (Matthew 7:6); 2 Pet. 2:2222But it is happened unto them according to the true proverb, The dog is turned to his own vomit again; and the sow that was washed to her wallowing in the mire. (2 Peter 2:22)). The prodigal had reached the lowest point of degradation when he would fain have satisfied his hunger with the swine’s food (Luke 15:1616And he would fain have filled his belly with the husks that the swine did eat: and no man gave unto him. (Luke 15:16)).

From Manners and Customs of the Bible:

This was considered one of the most degrading employments, not only by the Jews, but by other nations. Among the Egyptians, for example, the swineherd was completely shut out from society. The Saviour makes use of this antipathy to illustrate the depth of misery to which the dissipation of the young prodigal had brought him.

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