Charles Spurgeon Commentary Deuteronomy 32:5

"They have dealt corruptly with him, [they are] not his children, [it is] their blemish; [They are] a perverse and crooked generation." — Deuteronomy 32:5 (ASV)

Their spot is not the spot of his children: they are a perverse and crooked generation.

God's children have spots—the spot caused by sin—which are recognized, mourned over, and struggled against by them; the ungodly have the same sort of spots, but they have no repentance concerning the sin which causes them.

They have corrupted themselves, their spot is not the spot of his children: they are a perverse and crooked generation.

What a stoop from the God of truth, without iniquity, to a people full of iniquity – a perverse and crooked generation. We never know so much of our own vileness as when we get a clear view of the excellency of God. What said Job? I have heard of thee by the hearing of the ear, but now mine eye seeth thee, wherefore I abhor myself and repent in dust and ashes.