Those who die in the grace and friendship of God, but not completely clean, although sure of their eternal salvation, undergo a purification after death, in order to obtain the holiness necessary to enter the joy of heaven.
This doctrine is also based on the practice of praying for the dead, of which the Holy Scripture speaks. That is why Judas Maccabaeus made a sacrifice of atonement for the dead, so that they might be delivered from their sins (2 Mac 12, 46). From the earliest times, the Church has honored the memory of the dead and offered suffrages in their favor, especially the Holy Sacrifice of the Mass, so that they might be purified and reach the beatific vision of God. The Church also recommends alms, indulgences and works of penance in favor of the deceased.
Prayer of Saint Gertrude
Eternal Father, I offer You the Most Precious Blood of Thy Divine Son, Jesus, in union with the Masses said throughout the world today, for all the Holy Souls in purgatory, for sinners everywhere, for sinners in the universal Church, those in my own home and within my family. Amen.
Let us pray for the souls in purgatory!