5. They have corrupted themselves--that is, the Israelites by their
frequent lapses and their inveterate attachment to idolatry.
their spot is not the spot of his children--This is an allusion to the
marks which idolaters inscribe on their foreheads or their arms with
paint or other substances, in various colors and forms--straight, oval,
or circular, according to the favorite idol of their worship.
JFB.
Picture Study Bible