Purgatory is in fact DUE to the efficacy of Christ’s one-time sacrifice. It is the final sanctification/cleansing for those going to heaven. It is not a “last chance to get to heaven” as those who are going to heck aren’t going to experience purgatory, only those going to heaven are.
I have read were people thought a soul can lose heaven which is impossible. Why because it is the trip by God in this belief. No more chance for hell.