What is the doctrine of purgatory?

You have come to know the truth of a doctrine, much corrupted by the Roman Catholics, but denied altogether by Protestants—I mean the doctrine of Purgatory—or the intermediate state? How can I doubt it when I feel it? I have no doubt about it myself—not from your communications, but from reason and scripture. It was not the church of Rome that originated the doctrine—it had an earlier origin.

—Ruisdal or Steen in Spirit