Jeremiah 3:6

Those who place themselves far from your care and choose to serve idols will reap the destructive fruit of defection. He calls idolatry infidelity here. God likewise says also through Jeremiah, "She went up every high hill and under every green tree and was unfaithful there. I said, after all this infidelity of hers, 'Return unto me,' and she did not return." Again, "She committed adultery with tree and stone," meaning, "Leaving me, her spouse, she served false gods." Accordingly, here too he called the worship of idols infidelity.

It is written in the law: "You shall not commit adultery." This is rightly observed according to the simple meaning of the letter by one who is still in bondage to foul passions. But by one who has already forsaken these dirty acts and impure affections, it must be observed in the spirit, so that he may forsake not only the worship of idols but also all heathen superstitions and the observance of auguries and omens and all signs and days and times, or at any rate he should not be entangled in the conjectures of words and names that destroy the simplicity of our faith. This is the kind of fornication by which Jerusalem is said to have been corrupted, the fornication "on every high hill and beneath every leafing tree." The Lord criticized Jerusalem for this through the words of the prophet, "Let them stand and save you, these astrologers who studied the stars and counted the months so as to tell from these what was coming to you."

If, then, he says that first I sent away Israel because of its sins and sent it into deportation, but Judah, hearing what had happened to Israel, did not turn back, he speaks concerning our own sins. Since the things that befell Israel are read, and the transgressions concerning that people, we ought to fear, and: "if he did not spare the natural branches, how much more will he not spare us either; if that One who is both kind and at the same time loving to mankind, God, did not spare but cut off those who boasted to be a 'cultivated olive tree,' those rooted in 'the root' of the patriarchs Abraham and Isaac and Jacob, how much more will he not spare us." For he is not kind and yet not severe, nor severe and yet not kind. For if he were only kind but not severe, we would to a greater degree despise "his kindness." If he were severe but not kind, perhaps we would even despair over our sins. But as it is, as God — for we human beings need his kindness when we repent, but his severity when we persist in our sins — God is both kind and severe, and he speaks to us through the prophets and says: "Have you seen what the house of Israel has done to me?" Understand that Israel to be that people of mine: "She went up upon every high mountain and beneath every shady tree." If you see the Pharisee going up into the temple boastfully and not beating his own breast nor being anxious about his own evils, but saying: "I thank you that I am not as the rest of mankind, robbers, unjust, adulterers, or even as this tax-collector; I fast twice on the sabbath, I tithe my possessions" — you will see that he has gone up "upon every high mountain" in a blameworthy manner, having loved high-mindedness, and, according to his boasting and according to his arrogance, upon every high hill, and he has come to be "beneath every tree" that is not fruitful, but "leafy"; for the leafy tree is one thing, and another is the tree that men plant into groves. When they plant trees into groves, they plant not the fruit-bearing ones, not a fig tree nor a vine, but only barren trees for the sake of delight. Such you will find the words of the heterodox and the beauties of their persuasive arguments, which do not turn back those who hear. Whenever, then, someone gives himself over to such words, he has gone "beneath every leafy tree." He did not say "every tree" and fall silent, nor again did he add:…