Hosea 4:13

‘They sacrifice on the tops of the mountains, and burn incense on the hills, under oaks and poplars and terebinths, because its shadow is good. Therefore your daughters play the harlot, and your brides commit adultery.'

High places, (raised sites, especially in the hills), were ever a feature of Canaanite religion and Baal worship, and Israel had been under instructions to destroy them (Numbers 33:52). Furthermore Israel had been warned against such high places lest they come under God's anger (Leviticus 26:30). The tops of mountains were seen by idolaters as being nearest to the gods, which made them a favourite place for erecting high places. There they sacrificed and burned incense to their gods. Another favourite site for such high places was under green trees whose branches offered shade from the burning heat, those being selected which had widespread branches and thick foliage, of which examples are here given. They provided the ‘feel-good factor', and were also seen as containing ‘life' as demonstrated by their greenness. The specific identity of the trees is not certain.

The consequence of this worship was that the young Israelite women were introduced to illicit sex as a part of the religious ritual, with their young unmarried women acting like prostitutes, and their newly wed brides committing adultery. As a result the purity of the young women of Israel had become a thing of the past.

‘Because its shadow is good,' possibly a satirical indication that that is all the good that they can expect from it. It would not be what was in the mind of the worshippers. They probably thought in terms of its protectiveness (compare Psalms 91:1).