All Christians agree that we won’t be sinning in heaven. Sin and final glorification are utterly incompatible. Therefore, between the sinfulness of this life and the glories of heaven, we must be made pure. Between death and glory there is a purification.
The Catechism of the Catholic Church defines purgatory as a "purification, so as to achieve the holiness necessary to enter the joy of heaven," which is experienced by those "who die in God’s grace and friendship, but still imperfectly purified" (CCC 1030). It notes that "this final purification of the elect . . . is entirely different from the punishment of the damned" (CCC 1031).
The doctrine of hell is so frightening that numerous heretical sects end up denying the reality of an eternal hell. The Unitarian-Universalists, the Seventh-day Adventists, the Jehovah’s Witnesses, the Christadelphians, the Christian Scientists, the Religious Scientists, the New Agers, and the Mormons—all have rejected or modified the doctrine of hell so radically that it is no longer a serious threat.