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.
Nikolay Guryanov
Once he was asked what he considered to be the most dangerous sin in the souls of people? Father Nikolai answered: "Unbelief! This is frightening." - Even with Christians? - "Yes, even, with Orthodox Christians".