According to the Catechism of the Catholic Church, "All who die in Gods grace and friendship, but still imperfectly purified, are indeed assured of their eternal salvation, but after death they undergo purification, so as to achieve the holiness necessary to enter the joy of heaven."Purgatory comes from the Catholic faith and Christians don't believe in it because it is not biblical.
For Christians who base spiritual truth on the Bible alone, there are problems with the doctrine of purgatory. For example:
It is not explicitly found in the Bible.
It implies that the righteousness of Christ does not cleanse from all sin.
It implies that justification is not by faith alone.
It implies that there is something we must do in order to be cleansed of sin.