Each human person is endowed with an immortal soul from the moment of our conception. Our souls exist into eternity, and we are each particularly judged at the end of our life and assigned to hell or heaven, either immediately or after a period of purification.
Eternity in hell is the result of dying “in mortal sin without repenting and accepting God’s merciful love … remaining separated from him for ever by our own free choice” (CCC, No. 1033). If assigned to hell, one is eternally separated from God as the result of his or her own free will. God doesn’t want “any to perish, but all to come to repentance” (2 Pt 3:9).
Michael R. Heinlein is editor of Simply Catholic. Follow him on Twitter@HeinleinMichael.