EWTN Catholic Q&A
Question from PJ on 11-12-2012:

Fr. Eckert,

You mention purgatory is for EITHER unpaid temporal punishment due to sin OR a cleansing of desire or habit to sin. It is understandable that indulgences may be used to remit temporal punishment....but how is it that a plenary indulgence can possibly cleanse someone of their desire or habit (i.e. attachment) to sin. Please clarify how someone else's merits can make this happen.

Answer by Fr. John Echert on 11-20-2012:

In order to gain a plenary (full) indulgence, a person must be free from all attachment to sin. Less than that and the indulgence is partial. And subsequent to obtaining a plenary indulgence, if someone has an attachment to sin and dies, then the indulgence would not satisfy or atone for that subsequent attachment.

Thanks, PJ

Father Echert