: To offer intercession for those who died in God’s grace but still require cleansing from "lesser" or venial sins before entering the full presence of God in heaven.

The practice of praying for the dead dates back to early Christianity and the Old Testament (2 Maccabees). All Souls' Day - Article - The Abbot's Circle

The day is rooted in the belief that the prayers of the living can assist the souls of the deceased who are in a state of purification, often referred to as .

: Catholic doctrine views the Church as a single family in three states: the Church Triumphant (saints in heaven), the Church Militant (believers on earth), and the Church Suffering (souls in purgatory). Historical Origins