Thus saith the LORD, What iniquity have your fathers found in me, that they are gone far from me, and have walked after vanity, and are become vain?
Iniquity - wrong done to them (Isaiah 5:4; Micah 6:3; cf. Deuteronomy 32:4.)
Walked after vanity - contrasted with "wentest (walkest: the same Hebrew verb as here, Jeremiah 2:5) after me in the wilderness" (Jeremiah 2:2): then I was their guide in the barren desert; now they take idols as their guides.
Vanity ... vain. An idol is not only vain (impotent and empty), but vanity itself. Its worshippers acquire its character, becoming vain as it is (Deuteronomy 7:26; Psalms 115:8). A people's character never rises above that of its gods, which are its 'better nature' (Bacon). (2 Kings 17:15, "They followed vanity, and became vain;" Jonah 2:8, "They that observe lying vanities forsake their own mercy").