They have corrupted themselves, their spot is not the spot of his children: they are a perverse and crooked generation.
They have corrupted themselves - i:e., 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 colours and forms-straight, oval, or circular-according to the favourite idol of their worship. There is no reason to believe that the ancient people of God were ever distinguished by any visible marks on their persons of their devotion to His service. But they had witnessed those outward badges of idolatry on the pagan people with whom they had come in contact: so that the figurative language of the song would be universally understood to mean that their character and conduct were not such that observers would recognize in them any resemblance to the worshippers of the true God.
They are a perverse and crooked generation, х dowr (H1755) `iqeesh (H6141) uwptaltol (H6618)] - a false (deceitful), and crooked [from paathal (H6617), to twist], and twisted generation. Such being the unhappy state of Israel, the sacred bard expostulates with them, why they had made so base a return to God for all His benefits.