According to many saints: if the devil asked God for forgiveness it would be granted.
But the angels possess supernatural knowledge and understanding and there is nothing that could ever make them change their minds. Humans suffer from ignorance and weakness. Our decisions are inconstant but the angels’ decisions are not. They are not like us.
CCC 393. It is the irrevocable character of their choice, and not a defect in the infinite divine mercy, that makes the angels’ sin unforgivable. “There is no repentance for the angels after their fall, just as there is no repentance for men after death.”
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