Lord hath sworn by the excellency of Jacob - that is, by Himself, in whom Jacob’s seed glory [Maurer]. Rather, by the spiritual privileges of Israel, the adoption as His peculiar people [Calvin], the temple, and its Shekinah symbol of His presence. Compare Amo 6:8, where it means Jehovah’s temple (compare Amo 4:2).
never forget - not pass by without punishing (Amo 8:2; Hos 8:13; Hos 9:9).