To recap so far:
1) The souls in Purgatory are “the elect” and are assured Heaven.
2) Purgatory is a place of final purification to cleanse us of any unholy attachments or penance we still owed at our death.
So doesn’t that mean people just have to wait their time out in Purgatory until they are finished?
Yes and no.
Yes, if no one is praying for them or offering up penance for them the souls will remain in Purgatory until their “time” is up. It’s important to remember time is a relative concept since it’s been implied that (like Heaven) time is different in Purgatory than it is on earth.
However, we, the Church Militant, can do something for those in Purgatory to help them reach Heaven sooner. We can offer up something called Indulgences for the souls in Purgatory.

Whoa, wait a minute, the Church doesn’t believe in that anymore, right?!? That was why the Protestant Reformation happened!
Yes, some clergy back in the day misused the doctrine of Indulgences to raise money for the Church. They were wrong. But that doesn’t make Indulgences wrong, just how they were handled in the past. The Council of Trent addressed this in 1563. It called to amend and correct the abuses that crept into this practice by having all bishops report abuses of the practice as they saw them. The Council also stated “that this gift of holy Indulgences may be dispensed to all the faithful, piously, holily, and incorruptly.”
To sum up, the Council did not deny the existence of Indulgences, nor did it eliminate the practice of Indulgences; it only corrected the misuse and abuse of the practice.
If you look in some older Catholic books you may see a time attached to an Indulgence, e.g. 100 years “off of Purgatory” for performing a certain penance. That system was really to rank order various pious acts, rather than to guarantee something would knock, say, 100 years off of someone’s time in Purgatory. Partly because of the confusion it caused, that system no longer exists today. The two types of Indulgences are Plenary (meaning full or complete) and Partial (meaning, well, partial or not complete).
Only God will know if the Indulgence we offer meets all the requirements of a Plenary Indulgence. Even if we can’t perform the act of a Plenary Indulgence on a regular basis, there are plenty of Partial Indulgences we can offer up every day.
One way to gain a Partial Indulgence is to pray Psalm 130 (in some Bibles it’s Psalm 129):
Out of the depths I cry to You,
O Lord: Lord, hear my voice.
Let Your ears be attentive, to the voice of my supplication.
If You, O Lord, should mark iniquities, Lord, who could stand?
But with You there is forgiveness, that you may be revered.
My soul is waiting for the Lord; my soul has relied on His word.
My soul is longing for the Lord, more than sentinels wait for the dawn.
More than sentinels wait for the dawn, let Israel wait for the Lord.
For with the Lord there is mercy, and with Him is plentiful redemption.
And He shall redeem Israel from all his iniquities.
Today the soul I am praying for is Brian Bower.
“Lord have mercy on all who appear before You today.”

