“Because you have said, ‘These two nations and these two countries will be mine, and we will possess it', whereas Yahweh was there. Therefore as I live says the Lord Yahweh, I will act in accordance with your anger, and in accordance with your envy which you have shown in your hatred against them, and I will make myself known among them, when I judge you. And you will know that I, Yahweh, have heard all your blasphemies which you have spoken against the mountains of Israel, saying, ‘They are laid desolate, they are given to us to devour'. And you have magnified yourselves against me, with your mouth, and have multiplied your words against me. I have heard it.”
Edom were guilty of two major crimes. They considered that they could annex the land that belonged to Yahweh, His possession, and they had magnified themselves (and their gods) against Yahweh. The two nations here are Israel and Judah. But the land belonged to Yahweh. ‘Yahweh was there', as they well knew. Thus in saying what they did they were despising Yahweh.
Furthermore their acts were acts of anger and jealousy against their ‘brothers', acts which therefore drew on them Yahweh's retribution. Thus when the retribution came Israel and Judah would recognise in it the hand of Yahweh, ‘I will make myself known among them when I judge you'.
And their words against the mountains of Israel were blasphemy, because while those mountains were Yahweh's, they considered that they were now given to them (by their gods). The implication may also be that they had actually stated openly and constantly that Yahweh's land was now given to them and their gods. That would be the view of an ancient nation. And it would make the blasphemy even more outrageous.
‘I have heard it.' All is open to Him. No word can be spoken without Him hearing it. Thus He wanted them to know that He had overheard their words and would call them to account. Men are justified or condemned by their words (Matthew 12:37).