As a protestant one of my main contentions with the Catholic faith was their insistence on this transitory place called 'Purgatory'. And then I was pointed to the texts in Holy Writ that affirmed the teaching of the Church. Texts like the parable of Lazarus and the rich man (Luke 16:19-31) where we see the rich man in a place of torment. In this place of torment he is calling Abraham his father, and interceding for his brothers - probably not something a reprobate does in hell proper.
Then there is the passage in I Cor. 3 where a man whose works are as straw will be still be saved, but only through fire. I was convinced quite easily after being pointed to 1 John 1:9 that "IF we confess our sins, He will be faithful and just and forgive us our sins and PURIFY us from all unrighteousness." The key being 'IF'. What IF I didn't confess my sins and still died in a state of grace? According to First John, my sins are only forgiven if I confess them. Further, we need to be purified after being forgiven. Thus, a place of purgation simply made sense and wiped away this idea that we can sin a plenty and when we die it is all just magically wiped away without even being sorry for it.
The question that really interested me was why can I gain an indulgence for a soul in purgatory, but not for my wife or child or anyone in the Church Militant?
First we must answer what an indulgence is. My children always need a good reminding of this as the language is a bit foreign these days. I use the following example:
Let's say my son came into the house and sprayed ketchup all over the furniture and walls in a fit of mischief and anger. Then he came and with all sincerity begged our forgiveness. The forgiveness would be granted and his penance would be to clean up the ketchup, clean the furniture and to pay for what could not be cleaned. Simple. But let's say the ketchup stained the $1000 curtains and they couldn't be cleaned and there was no way he could pay the debt (with his minimal allowance) on these curtains except over many many years. As a father, I could grant him an indulgence and have him pay $50 for their replacement and pay the rest myself - simply because he is my son and I love him.
The Church, having the keys the heaven and earth, does the same. The Church grants indulgences to us to cover the temporal (not eternal) consequences of our sins that are or are not being paid for with our penance. In her mercy, as a Mother, she helps to pay the debt owed.
So why can I gain an indulgence for a soul in purgatory?
The answer is joyously simple.
Those of us in the Church Militant are hopefully in a state of grace and in various degrees of openness to receiving grace depending on our situation in life. Some days, I may struggle with sin and not be disposed to receiving God's grace and other days I may be on my face begging for His grace. Those, in purgatory however, are always in the disposition to receive God's grace via an indulgence. Thus, we can pray and do the required actions for an indulgence to be granted and it will be granted and received.
What must be done to receive an indulgence during this octave of All Souls, is to pray in a cemetery for the faithful departed plus the regular conditions of praying devoutly for the Pope's intentions, the Our Father, the Hail Mary, Glory be, and the Creed. As well, receiving the Holy Eucharist at Mass and the Sacrament of confession. And - the hardest of all - being free from all attachment to original sin.