Charles Spurgeon Commentary Isaiah 57:5

"ye that inflame yourselves among the oaks, under every green tree; that slay the children in the valleys, under the clefts of the rocks?" — Isaiah 57:5 (ASV)

The Lord had said that they should offer sacrifice only on one altar at Jerusalem, and this to him alone, but they had set up altars under all the ancient oaks to worship all sorts of gods. In addition to this, they had gone so far in following the cruel way of the Pagans, that they offered their own children in sacrifice in the valleys, under the cliffs and the rocks.