Why is the sin of the fallen angels unforgivable? (Catholic catechism) Referring to the fall of Satan and his angels, according to the Catechism:


  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."


Can someone please explain further why the angels who turned away from God cannot be forgiven, unlike man? What does irrevocable character of their choice mean?
Satan and all the fallen angels (demons) are completely consumed with evil and chose to do evil with all their intellect and sin against God, thus repentance is no longer possible for them.


  According to Canon 1 of the VI Lateran Council in 1215, the fallen angels were good. Does any goodness still exist within these demons and can they repent since God is merciful to sinful humans who repent of their evil? According to the teaching of the Catholic Church, no. There is not good left within Satan and his angels and they are completely consumed by evil. As a result of this and because they chose to sin against God with their full intellect and will. As Paragraph 393 of the Catechism of the Catholic Church states:
  
  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." - The Catholic Teachings On The Angels - Part 2: The Fall Of Satan


Before the death of a human being, man is able to repent of past sins, but not after death.

Why does the soul become immutably fixed, in good or in evil, immediately after death can be read about here:  IMMUTABILITY AFTER DEATH.