The hardest part of getting a plenary indulgence is not having any attachment to sin.
What does this mean? Example: A young man and woman are at the altar, getting married. The priest asks the man, “Do you take this woman…etc.” He immediately says, “I do!” Then, he turns to the woman and asks, “Do you take this man…, etc.” and the woman is kind of looking off in the distance with a glaze in her eyes. Everyone is waiting, so the groom clears his throat and she comes back to reality, and says, “Oh, yes. I do.” Afterward, when they’re in the limo heading towards the reception, he asks her about that, why she seemed distracted, and she says, “Oh, we’re married and everything and I will love you forever. But at that moment, I was thinking about my previous boyfriend and how fantastic we had it together, and how wonderful he was. But that didn’t work out, so I’m yours now.” How do you think that young man felt then? God feels the same way when we have an attachment to sin, something that’s sinful that we just really would prefer not to give up.