Albert Barnes Commentary Jeremiah 2:28

"But where are thy gods that thou hast made thee? let them arise, if they can save thee in the time of thy trouble: for according to the number of thy cities are thy gods, O Judah." — Jeremiah 2:28 (ASV)

A question of bitter irony. Things are made for some use. Now is the time for your deities to prove themselves real by being useful. When every city has its special deity, surely among so many there might be found one able to help his worshippers.

O Judah—Until now the argument had been addressed to Israel: suddenly the prophet charges Judah with the habitual practice of idolatry, and points to the conclusion, that as Jerusalem has been guilty of Samaria’s sin, it must suffer Samaria’s punishment.