John Calvin sums up the essence of the doctrine of purgatory:
"For what means this purgatory of theirs but that satisfaction for sins is paid by the souls of the dead after their death?"
Then he sums up the primary problem with purgatory:
"But if it is perfectly clear ... that the blood of Christ is the sole satisfaction for the sins of believers, the sole expiation, the sole purgation, what remains but to say that purgatory is simply a dreadful blasphemy against Christ?"
--Institutes of the Christian Religion (3.5.6)