Why are Souls in Purgatory?

In order to understand why the Poor Souls are in purgatory, as Catholics we should know what we believe about the double effect of every sin. Every time we sin we lose more or less of God's grace. This we call "guilt." Every time we sin we also incur a debt of pain. This we call "penalty."
Mortal sins are called mortal because they deprive the sinner of sanctifying grace and the right to heaven. Mortal sins also incur the debt of eternal punishment.

When mortal sins are forgiven in the sacrament of penance, sanctifying grace is restored and the debt of eternal punishment is removed. But normally there is still temporal punishment to be expiated. Moreover, venial sins always carry with them a debt of temporal punishment, which is more or less remitted, depending on a person's spiritual dispositions.

This immediately tells us that because we are sinners we must expect to suffer in expiation for our sins. The choice we have is between patiently suffering here on earth or suffering in purgatory after our bodily death.

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