On the Life of Moses: Books 1-3 (1.301-1.305)

Accordingly, when licence was thus given, they brought over a multitude of young men, having already long before this seduced their minds, and having by their tricks and allurements perverted them to impiety; until Phinehas, the son of the chief priest, being exceedingly indignant at all that was taking place (for it appeared to him to be a most scandalous thing for his countrymen to give up at one time both their bodies and souls—their bodies to pleasure, and their souls to

For when he saw a man of his nation sacrificing with and then entering into the tent of a harlot, and that too without casting his eyes down on the ground and seeking to avoid the notice of the multitude, but making a display of his licentiousness with shameless boldness, and giving himself airs as if he were about to engage in a creditable action, and one deserving of smiles—Phinehas, I say, being very indignant and being filled with a just anger, ran in, and while they were still lying on the bed, slew both the lover and the harlot, cutting them in two pieces in the middle, because they thus indulged in illicit connections.

When some persons of those who admired temperance, and chastity, and piety, saw this example, they, at the command of Moses, imitated it, and slew all their own relations and friends, even to a man, who had sacrificed to idols made with hands, and thus they effaced the stain which was defiling the nation by this implacable revenge which they thus wreaked on those who had set the example of wrong doing, and so saved the rest, who made a clear defence of themselves, demonstrating their own piety, showing no compassion on any one of those who were justly condemned to death, and not passing over their offences out of pity, but looking upon those who slew them as pure from all sin. Therefore they did not allow any escape whatever to those who sinned in this way, and such conduct is the truest praise;

and they say that twenty-four thousand men were slain in one day, the common pollution, which was defiling the whole army, being thus at once got rid of. And when the works of purification were thus accomplished, Moses began to seek how he might give an honour worthy of him who had displayed such permanent excellence to the son of the chief priest, who was the first who hastened to inflict chastisement on the offenders. But God was beforehand with him, giving to Phinehas, by means of his holy word, the greatest of all good things, namely, peace, which no man is able to bestow; and also, in addition to this peace, he gave him the perpetual possession of the priesthood, an inheritance to his family, which could not be taken from it.

But when none of the civil and intestine evils remained any longer, but when all the men who were suspected of having either forsaken the ways of their ancestors or of treachery had perished, it appeared to be a most favourable opportunity for making an expedition against Balak, a man who had both planned to do, and had also executed an innumerable host of evil deeds, since he had planned them through the agency of the prophet, who he hoped would be able, by means of his curses, to destroy the power of the Hebrews, and who had executed his purpose by the agency of the licentiousness and incontinence of the women, who destroyed the bodies of those who associated with them by debauchery, and their souls by impiety.