I’ve always heard that Purgatory isn’t so much a “place” as it is a “process” of purification. It is the process of ‘detaching’ ourselves of our ‘desire’ to sin. Many of us start this process here on earth. We do that by putting God first in our lives, loving Him and loving our neighbor, and repeatedly examining our conscience, trying to apply the message of the gospel to our lives. I’ve always heard that when we die, we die in full conscience of who we are. So when we die, even though our sins are forgiven, we might not have detached ourselves from the weaknesses that caused us to sin. Maybe we haven’t completely let go of our pride, or our selfishness, or an anger. I believe there is a passage in Revelation that says nothing impure can ever enter heaven. So if we die with a few imperfections, we need to resolve those before we enter the gates of heaven.
Purgatory doesn’t run counter to the gospel as I see it. I think it shows both God’s mercy and His justice. He forgives our sin and therefore doesn’t condemn us to an eternity of hell, but still does not allow us immediate access to Heaven. That actually makes sense to me. Growing spiritually is really hard work, I think. It’s not easy to draw away from the desires of this world and seek communion with God. It’s would be easier to stay angry than offer forgiveness. It would be easier to treat other people the way they treat me, rather than treat them the way I want to be treated. If I don’t spend my time on earth trying to live by these principals, how could I be ready for Heaven without spending some time in a process of purification? Basically isn’t that what our lives are all about - practicing for eternity?
I think hell is a place for those who totally reject God. Heaven is for those who have learned perfect love of God and neighbor. And purgatory is for those who are in various stages of learning to love as God loves. If we don’t finish this process before we die, God allows us to finish it after we die.
Sometimes, I think, as Christians, we focus so much on God’s mercy (which is awesome) that we forget that He is also a God of justice. His mercy exceeds His justice to be sure, but that doesn’t mean He applies mercy at the expense of justice, if that makes any sense.
Hope this helps. I’m still in the process of trying to learn how to explain our faith! And like the poster before me, I’m sure others on this forum can explain it much better.