Purgatory does not deny true efficacy of the cross. Your comment begs the question, what did Christ means by “it is finished” ? His mission,of course, which is the salvation of the world. But the world continues, and the work of the devil continues. For which the minions of the devil reproach us, by pointing out the ways we fall short but also that Christ has not come again. Yet we die, one by one, and it is only by dying that we see the course of our life, and how many of us see a perfect trust in Him?