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:
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".