Eternal sin
Thomas Aquinas lists, or has responded to, six sins that supposedly go against the Holy Spirit:
Thomas Aquinas explains that the unforgivability of blasphemy against the Holy Spirit means that it removes the entrance to these means of salvation; however, it cannot hinder God in taking away this obstacle by way of a miracle.
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.