Eternal sin
In Catholic teaching there are six sins that blaspheme against the Holy Spirit. They are: Several passages in the Bible are frequently interpreted as referring to the unforgivable sin:
Pliny the Younger on Christians
Neither Pliny nor Trajan mentions the crime that Christians had committed, except for being a Christian; and other historical sources do not provide a simple answer to what that crime could be, but most likely due to the stubborn refusal of Christians to worship Roman gods; making them appear as objecting to Roman rule.