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.
Theology of Søren Kierkegaard
Sin is separation from God but despair over sin is separation again. Kierkegaard said, "The consciousness of sin definitely belongs to the consciousness of the forgiveness of sin." Why would someone sit and reflect on sin to such an extent that an eternal happiness is exchanged for an eternal unhappiness or even a temporal unhappiness? This reflection is done in time but the consequence of the reflection leads one to lose hope in the possibility of any good coming from oneself. Kierkegaard says Christianity invites the single individual to become a partaker not only of the consciousness of sin but also of the consciousness of forgiveness but we seem to concentrate on the former to a remarkable degree. He said the following in "Three Discourses on Imagined Occasions" (1845) and "Practice in Christianity" (1850):