In the words of John Miley:

 

"Purgatory, as an assumed Christian doctrine, is peculiar to Romanism.  It has no place in the creed of any other Church, though in some it may be held by individual members.  In Romanism Christians compose two classes: the imperfect, and the truly good.  The former have impurities which must be cleansed away, and venial sins which must be expiated in penal suffering, in order to a meetness for heaven.  Even the truly good, while free from the guilt of mortal sins, yet have deserts of temporal punishment which must be expiated.  Purgatory provides  for both classes, as in its penal and purifying fires both may attain to a fitness for heaven.  But it provides only for such as the Romish Church recognizes as Christians; therefore it has no connection with the doctrine of a second probation.

It is part of the doctrine that purgatory is in some respects subject to the Church.  By prayers, and alms, and masses its penal sufferings may be mitigated, or the hour of release hastened…It may be found in Homer, and Plato, and Virgil, and other classical writers, but not in the Scriptures.  It was unknown to the early Church; assumed no definite form until late in the fourth century; and was first decreed as an article of faith by the Council or Florence in the fifteenth century.  The doctrine is openly false to the soteriology of the Gospel, according to which we are saved, completely saved, from the guilt and pollution of sin through the blood of Christ and the sanctification of the Spirit."

(Miley, v2, p 438-439)