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.
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").