Son of perdition is a phrase used in The Church of Jesus Christ of Latter-day Saints (LDS Church) and by Mormon fundamentalists to describe a person who will not take part in the glory of God in the afterlife. This is in contrast to the vast majority of people, who will receive a "kingdom of glory" after the Final Judgment, and enter into one of three degrees of glory after the resurrection: celestial, terrestrial, or telestial kingdoms.
Most Latter-day Saints believe that the sons of perdition will be cast into outer darkness; Mormon scriptures do not use this exact phrase in connection with the sons of perdition, but state that they "shall go away into the lake of fire and brimstone, with the devil and his angels."
Latter-day Saints believe that free will is one of the greatest gifts of God, and that there might be some who will completely reject Jesus Christ and salvation. God will not force someone, including Satan, to be saved if they do not desire to be saved. Therefore, the sons of perdition are permitted to remove themselves entirely from the presence of God.
In this context, the name "Perdition" is sometimes regarded as a proper name that refers to either Lucifer or Cain, both of whom are symbols of ultimate evil.
The most comprehensive exposition of the phrase is found in Section 76 in the Doctrine and Covenants.
According to LDS Church theology, there are two classes of persons who will become sons of perdition:
All sins shall be forgiven, except the sin against the Holy Ghost; for Jesus will save all except the sons of perdition. What must a man do to commit the unpardonable sin? He must receive the Holy Ghost, have the heavens opened unto him, and know God, and then sin against him. After a man has sinned against the Holy Ghost, there is no repentance for him. He has got to say that the sun does not shine while he sees it; he has got to deny Jesus Christ when the heavens have been opened unto him, and to deny the plan of salvation with his eyes open to the truth of it.