Using Hebrews as the exegetical point; Couldnt the argument be that apostacy is the unforgivable sin. Not despair? Despair, as an emotion. Is temporary in nature. Depression is a horrible thing, but I would not consider it a sin, rather a fault in a fallen world.I believe the Hebrews passage you are referring to is from Ch. 6:
4 For it is impossible to restore again to repentance those who have once been enlightened, who have tasted the heavenly gift, and have become partakers of the Holy Spirit,
5 and have tasted the goodness of the word of God and the powers of the age to come,
6 if they then commit apostasy, since they crucify the Son of God on their own account and hold him up to contempt.
7 For land which has drunk the rain that often falls upon it, and brings forth vegetation useful to those for whose sake it is cultivated, receives a blessing from God.
8 But if it bears thorns and thistles, it is worthless and near to being cursed; its end is to be burned.
Seeing this (and also Heb 10:26-39), I think one could rightly add "apostasy" to the list of sins that are unforiveable insofar as they lead one to final impenitence. However, I would not include it with final impenitence as a sin that is absolutely unforgiveable.
Yes, Paul does say that "it is impossible to restore again to repentance those who have once been enlightened....if they then commit apostasy," but what he has in mind is the state of the apostate for as long as he remains separated from the Church. As long as a man remains in a state of apostasy, he cannot be forgiven because he is willfully living a life in rejection of the Church and her teaching. Such a man surely cannot be forgiven for the simple fact that he is not seeking forgiveness, and he has separated himself from the very Sacraments that insure his forgiveness. But, if he turns away from apostasy and embraces the Church again, he will be surely taken in and forgiven. Paul has in mind the obstinant man, not the contrite one.
As for despair, it is much more than an "emotion" or a part of our fallen world. It is a sin against hope. This is what the Catechism says about despair:
2091 The first commandment is also concerned with sins against hope, namely, despair and presumption:A few Catholic reference works are also helpful here:
By despair, man ceases to hope for his personal salvation from God, for help in attaining it or for the forgiveness of his sins. Despair is contrary to God's goodness, to his justice - for the Lord is faithful to his promises - and to his mercy.
1910 New Catholic Dictionary:
Despair -- (Latin: desperare; to lose hope) Psychologically, the passion contrary to hope; morally, the abandonment of all hope of salvation or of the means required for it; not merely an anxiety about one's future state, or fear that one may be lost, but a deliberate yielding to the conviction that human nature cannot cooperate with God's grace, that one's sins are unpardonable, or that Almighty God has cast one away. It is an offense against God's goodness and mercy, temptation to which should be resisted not only for moral but for physiological reasons also, since it commonly results in melancholy, or in sinful indulgence.Catholic Glossary:
Despair -- Abandonment of hope for salvation arising from the conviction that God will not provide the necessary means for attaining it, that following God’s way of life for salvation is impossible, or that one’s sins are unforgivable; a serious sin against the Holy Spirit and the theological virtues of hope and faith, involving distrust in the mercy and goodness of God and a denial of the truths that God wills the salvation of all persons and provides sufficient grace for it. Real despair is distinguished from unreasonable fear with respect to the difficulties of attaining salvation, from morbid anxiety over the demands of divine justice, and from feelings of despair.Pocket Catholic Dictionary:
Despair -- The sin by which a person gives up all hope of salvation or of the means necessary to reach heaven. It is therefore not mere anxiety about the future or fear that one may be lost. It is rather a deliberate yielding to the idea that human nature cannot co-operate with God's grace, or that the despairing person is too wicked to be saved, or that God has cast one away. It is a grave crime against God's goodness. Experience also shows that a tendency to despair can seriously injure one's physical and mental health, and ironically can lead to all kinds of sinful indulgence. (Etym. Latin de, the opposite of + sperare, to hope: desperatio, hopelessness, despair.)As you can see, despair is a very serious matter. It is more than just depression, or sadness, or perhaps some lingering doubts about God's love for you (we are all inclined to such feelings at one time or another). Despair has an element of persistence, and a dogged refusal to believe that God can do anything to save you, help you, heal you, etc. It is a denial of the goodness and power of God. As such, it is most certainly sinful.
I hope that helps.
ps: for the sake of clarification, I also slightly modified my descriptions of "obstinancy" and "impenitence" in my previous post.