The real basis for the doctrine of purgatory is neither scripture nor tradition, but the 'church's practice of prayer and penance'. Since the beginning--so argument runs--there have been in the church prayers for the dead, good works, almsgiving, personal penitential practices, and the acquisition of indulgences, vicariously applicable to the dead, which free them from punishments for sin. The Letter of the Congregation for the Doctrine of the Faith on Certain Questions of Eschatology (17 May 1979) puts it event more clearly: 'The church rejects all ways of thinking and speaking through which its prayers, the burial rites and the cult of the dead would lose their meaning and become incomprehensible: for all this is in substance a locus theologicus.' But that means in plai term that theology is there in order to justify the existing practice of the church. Once this method is followed, there is no possible way of examining particular ecclesiastical and devotional practices for their conformity to scripture and gospel.
Jurgen Moltmann, The Coming of God: Christian Eschatology, 99.