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:
Blasphemy
Blasphemy has been condemned as a serious sin by the major creeds and Church theologians (apostasy and infidelity [unbelief] were generally considered to be the gravest sins, with heresy a greater sin than blasphemy, cf. Thomas Aquinas' "Summa Theologiae").