Under oaks - אלון allon, from אלל alal, he was strong. Hence, the oak, in Latin, is called robur; which word means also, strength, the oak being the strongest of all the trees of the forest.
The shadow thereof is good - Their "daughters committed whoredom, and their spouses committed adultery."
3. The thick groves were favorable to the whoredoms and adulteries mentioned here. In imitation of these, some nations have their public gardens.
They sacrifice upon the tops of the mountains - The tops of hills or mountains seemed nearer heaven, the air was purer, the place more removed from the world. To worship the Unseen God upon them, was then the suggestion of natural feeling and of simple devotion. God Himself directed the typical sacrifice of Isaac to take place on a mountain; on that same mountain He commanded that the temple should be built; on a mountain, God gave the law; on a mountain was our Saviour transfigured; on a mountain was He crucified; from a mountain He ascended into heaven. Mountains and hills have accordingly often been chosen for Christian churches and monasteries. But the same natural feeling, misdirected, made them the places of pagan idolatry and pagan sins. The Pagan probably also chose for their star and planet-worship, mountains or large plains, as being the places from where the heavenly bodies might be seen most widely.
Being thus connected with idolatry and sin, God strictly forbade the worship on the high places, and (as is the case with so many of God‘s commandments) man practiced it as diligently as if He had commanded it. God had said, “Ye shall utterly destroy all the places, wherein the nations, which ye shall possess, served their gods upon the high mountains, and upon the hills and under every green tree” Deuteronomy 12:2. But “they set them up images and groves (rather images of Ashtaroth) in every high hill and under every green tree, and there they burnt incense in all the high place, as did the pagan whom the Lord carried away before them” 2 Kings 17:10-11. The words express, that this which God forbade they did diligently; “they sacrificed much and diligently; they burned incense much and diligently”; and that, not here and there, but generally, “on the tops of the mountains,” and, as it were, in the open face of heaven. So also Ezekiel complains, “They saw every high hill and all the thick trees, and they offered there their sacrifices, and there they presented the provocation of their offering; there also they made their sweet savor, and poured out there their drink-offerings” Ezekiel 20:28.
Under oaks - (white) poplars and elms (probably the terebinth or turpentine tree) because the shadow thereof is good The darkness of the shadow suited alike the cruel and the profligate deeds which were done in honor of their false gods. In the open face of day, and in secret they carried on their sin.
Therefore their daughters shall commit whoredoms, and their spouses - (or more probably, daughters-in-law) shall commit adultery Or (in the present) commit adultery. The fathers and husbands gave themselves to the abominable rites of Baal-peor and Ashtaroth, and so the daughters and daughters-in-law followed their example. This was by the permission of God, who, since they “glorified not” God as they ought, “gave them up,” abandoned them, “to vile affections.” So, through their own disgrace and bitter griefs, in the persons of those whose honor they most cherished, they should learn how ill they themselves had done, in departing from Him who is the Father and Husband of every soul. The sins of the fathers descend very often to the children, both in the way of nature, that the children inherit strong temptations to their parents‘ sin, and by way of example, that they greedily imitate, often exaggerate, them. Wouldest thou not have children, which thou wouldest wish unborn, reform thyself. The saying may include too sufferings at the hands of the enemy. “What thou dost willingly, that shall your daughters and your daughters-in-law suffer against thine and their will.”