Jamieson, Fausset and Brown on Amos 4:12

12. Therefore--as all chastisements have failed to make thee "return unto Me."

thus will I do unto thee--as I have threatened (Am 4:2, 3).

prepare to meet thy God--God is about to inflict the last and worst judgment on thee, the extinction of thy nationality; consider then what preparation thou canst make for encountering Him as thy foe (Jr 46:14; Lu 14:31, 32). But as that would be madness to think of (Is 27:4; Eze 22:14; He 10:31), see what can be done towards mitigating the severity of the coming judgment, by penitence (Is 27:5; 1Co 11:31). This latter exhortation is followed up in Am 5:4, 6, 8, 14, 15.