Matthew Poole and continuators on Genesis 14:13

13 And there came one that had escaped, and told Abram the Hebrew , for he dwelt in the Plain of Mamre the Amorite, Brother of Eshcol, and Brother of Aner: And these were confederate with Abram .

So called, either, 1. From his great and good Predecessour Eber, Gen. 10. 24. and 11. 14. in and by whom the primitive Language and true Religion were preserved, and therefore, though Abram had five other Progenitors between Eber and him, which were persons of less note, he is rightly denominated from Eber, the Hebrew, because he was the first that revived the memory and the work of Eber, that kept up the same Language, and eminently propagated the same true Religion. Or 2. As others think from his passing over the River Euphrates, from beyond which he came into Canaan.

i. e. Had entred into a League for their mutual defence against common Enemies. Whence we learn that it is not simply and universally unlawful to make a League, with persons of a false Religion.