The first thing that I did was to try to look up some Catholic teaching on the subject.
There's the Catechism of the Catholic Church.
And the Catholic Encyclopedia.
But those weren't really giving me the explanation I needed. I mean, they're good explanations, and if you've never heard of purgatory before, it's a good idea to read those so that you can get a feel for what the Catholic Church teaches on the subject. My problem is that these explanations are so sterile and clinical sounding. It doesn't really capture the extent of what I believe about purgatory or why I believe it. I certainly didn't feel that it answered the question.
I guess the main thing that I need to tell you is that I see spiritual life as a process. Actually, I see it as being a lot like the physical therapy process. If you come see me with upper back or neck pain, we're going to take a look at a lot of different things. One of those, of course, is posture. Bad posture starts a whole chain of crazy events. Let's say you like to sit at a desk with your shoulders slumped. If you do it once, it's not a big deal, but if you work at a computer 8 hours a day, 5 days a week, and slump you shoulders the whole time it's going to be an issue. The muscles that hold your shoulder blades back are going to get long and weak. When that happens, they may start to spasm in protest of trying to hold up your heavy arms (even skinny arms are heavy for weak muscles). The muscles in front (the pecs) are going to get stiff and tight. Then if you try to pull the shoulder blades back with your weak and wimpy back muscles, you're trying to pull against extra tight front muscles. When your shoulders are rounded forward, it leads to the arm bones lining up differently and pulling you forward even more. And the neck! Oh, your poor neck. The neck comes forward, but since you can't see if your head is down, then you bring your face up level with the world. That means that the bottom of your neck is too flexed and the top part of your neck is too extended. And another whole set of muscles becomes imbalanced, weak on one side, and stiff on the other.
The first time you sit in this posture, it will affect you. If you turn your head, it will not move like it's supposed to and stuff will rub that isn't supposed to and you will cause some damage. You won't notice that damage. It's only going to affect a few cells, which your body will be able to repair quickly. Do it over and over again, though. Do it for years. The gradual build up will lead to all kinds of fun stuff. Arthritis, compression of the nerves, to name only a couple of things.
What took you years to do will not be undone in a week. A week or two could make a difference, but there's still a lot to do. Also, it will not be fixed in physical therapy alone, each person has the responsibility to actually do the exercises and make the changes. Otherwise, they will get nowhere.
There it is. The spiritual life. The first time that I sin, it may not do to much. It will cause some damage, but it won't be all that noticeable. The more I sin, the more it will affect me, the more it will cause habits that lead to imbalances in the way that I see God and the world. This is going to lead to damage that could cause big problems.
When I first correct someone's posture, they can often hold the better posture, but it takes work. The first time that they correct the posture, it does not make all those muscle changes go away. As they work on the exercises, gradually weak muscles get strong and tight muscles get stretched out.
God immediately forgives my sins when I am sorry, but He allows me to be part of the process of correcting the imbalances that are left. I have to do the work and make the changes. This is a very important part of His grace, not a place that I have to fill in the gaps where the grace ran out. We all know that we have much more appreciation for something that we had to put our blood, sweat, and tears into than we do for something that was just handed to us. God allows us, with free will, to choose Him freely and to take part in the struggle so that we truly have a vested interest in what is going on.
With this view in mind, this is why purgatory makes complete sense to me. It's the gym where we get to work out the last areas of weakness. In other words, it's the place where we get to get rid of any of the areas that we are still holding onto our worldly, fleshly view of things and replace them with Christ. I think that the reason that we struggle with the idea of purgatory is that we have an instantaneous view of God's grace, rather than seeing the entire process as an outpouring of His grace from the cross.
Finally, I leave you with this. Whatever I am able to do, I am able to do through Christ. It isn't that I can do some things poorly without Him, I can do NOTHING without Him. However, in Him who strengthens me, I can do all things, even participate in working out my salvation. (Phil. 2:12) All the things that I do in Him is a foundation of gold and precious jewels. All things that I do without His grace is hay and straw. Purgatory is simply the refiner's fire that burns away all that is left of me that has not been touched by Him. (1 Cor 3:11-15).