Christians have believed in the Purgatory from the earliest times, which can be seen in the writings of the early Church. There wasn't any serious opposition to the doctrine until the Protestant Reformation in the 16th Century. The opposition is the result of a misunderstanding of what Purgatory really is.

Most Protestants would agree that unrepented sin cannot be brought into Heaven and that we must be sanctified before we enter it. This purification would occur instantly upon death. They have not given a name for this purification process, but Catholics call it Purgatory.