“Paradise” as spoken of by Christ on the Cross to the Good Thief was not heaven. That would have put the good thief in heaven before Christ — because Christ was in the tomb for three days.
Have not you read the verses where the dead arose from their graves and roamed around Jerusalem, visible to only the believers?
They, too, were waiting for Christ to be the first to enter heaven.
Thus, paradise was a waiting place, a purifying place, as is Purgatory.
When a person is in Purgatory they know they will reach heaven, but they must undergo the waiting and purification for the sins against others and the harm those sins did. Sort of a reparation.
But he was pierced for our transgressions,
he was crushed for our iniquities;
the punishment that brought us peace was on him,
and by his wounds we are healed.
-- Isaiah 53:5
The body of Christ was placed in the tomb for three days during which his spirit, according to the Nicene Creed, descended into Hell. Likely he also ascended into heaven. His spirit could have accompanied the thief on the cross to his entrance in heaven. Yet, Who knows? Maybe his Father demanded he spend three days in purgatory. Anything is possible, especially if you are a Catholic.
That’s a good explanation of how the promise to the thief can be reconciled with the concept of Purgatory.
I’ve thought myself that it’s also possible he went directly to Heaven (after it’s gates were open) as his time on his cross could be thought of as his own purgation. IOW, it’s entirely possible that his suffering then served to purge him of any attachment to sin he had. Indeed, for him to acknowledge his sin to Christ so freely indicates, at least to me, his attachment to sin at that point was quite low, if not zero. Also, the other theif provides a contrast to this, as he continued to mock Christ (thus still had an attachment to sin).
This concept is, I believe, quite in line with Catholic teaching; I believe it’s entirely acceptable to believe someone may have endured sufficient purgation here on Earth either by their own device or suffering imposed by another. (Terri Schiavo comes to mind here). Thus such people would go directly to Heaven.