Isaiah 57:5 - Matthew Poole's English Annotations on the Holy Bible

Inflaming yourselves with idols, Heb. Being inflamed, &c.; lusting after them, and mad upon them, as the phrase is, Jeremiah 50:38; fervent both in making and in worshipping of them, as was observed, Isaiah 44:12, &c. It is a metaphor borrowed from whoredom, to which idolatry is oft compared. Under every green tree; wheresoever you see an idol erected, which was commonly done in groves, or under great and shady trees, which both defended the worshippers from the heat of the sun, and were supposed to strike them with a kind of sacred horror and reverence. See Poole on "Deuteronomy 12:2"; See Poole on "2 Kings 16:4"; See Poole on "2 Kings 17:10". Slaying the children, in way of sacrifice to their idols, after the manner of the barbarous heathens; of which see on Leviticus 18:21 Deuteronomy 12:31 2 Kings 21:6, 2 Kings 23:10. In the valleys; or, beside the brooks, which run in the valleys; which was most commodious for such bloody work. He seems to allude to the valley of Hinnom, in which these cruelties were practised, Jeremiah 7:31, through which also the brook Kidron is supposed to have run. Under the clifts of the rocks; which they chose either for shade, or for those dark vaults and hollow places which were either by nature or art made in rocks, and which were convenient for and frequently appointed to idolatrous uses.