“Only you have I known from all the families of the earth; therefore I will punish you for all your iniquities.”
The Prophet afterwards declares what he had in charge, Only you have I known of all the families of the earth: I will therefore visit you for your iniquities. Many think that he still concedes to the Israelites what they were wont to boast of, — that they were separated from the common class of men, because the Lord had adopted them: but it seems...
The scope of these verses is to convince the people of Israel that God had a controversy with them. That which the prophet has to say to them is to let them know that the Lord has something to say against them, Amo 3:1.
Commenting on Amos 3:1-8
You only have I known of all the families of the earth,.... All the families or nations of the earth, and all the inhabitants of it, are known by the Lord, as he is the omniscient God; but none had been known by him as a family, or a nation, with that love and affection as this family had been, or distinguished by his favours...