31 And so I tell you, every kind of sin and slander can be forgiven, but blasphemy against the Spirit will not be forgiven. 32 Anyone who speaks a word against the Son of Man will be forgiven, but anyone who speaks against the Holy Spirit will not be forgiven, either in this age or in the age to come. Matthew 12:31-32 New International Version
That, I believe, is the scripture that they use to justify their position. They consider apostasy to be a crime against the holy spirit but anything else can be forgiven.