Objection 1. It would seem that despair is not the greatest of sins. For there can be despair without unbelief, as stated above (Article 2). But unbelief is the greatest of sins because it overthrows the foundation of the spiritual edifice. Therefore despair is not the greatest of sins.
Objection 2. Further, a greater evil is opposed to a greater good, as the Philosopher states (Ethic. viii, 10). But charity is greater than hope, according to 1 Corinthians 13:13. Therefore hatred of God is a greater sin than despair.
Objection 3. Further, in the sin of despair there is nothing but inordinate aversion from God: whereas in other sins there is not only inordinate aversion from God, but also an inordinate conversion. Therefore the sin of despair is not more but less grave than other sins.
On the contrary, An incurable sin seems to be most grievous, according to Jeremiah 30:12: "Thy bruise is incurable, thy wound is very grievous." Now the sin of despair is incurable, according to Jeremiah 15:18: "My wound is desperate so as to refuse to be healed." [Vulgate: 'Why is my wound,' etc.] Therefore despair is a most grievous sin.
I answer that, Those sins which are contrary to the theological virtues are in themselves more grievous than others: because, since the theological virtues have God for their object, the sins which are opposed to them imply aversion from God directly and principally. Now every mortal sin takes its principal malice and gravity from the fact of its turning away from God, for if it were possible to turn to a mutable good, even inordinately, without turning away from God, it would not be a mortal sin. Consequently a sin which, first and of its very nature, includes aversion from God, is most grievous among mortal sins.
Now unbelief, despair and hatred of God are opposed to the theological virtues: and among them, if we compare hatred of God and unbelief to despair, we shall find that, in themselves, that is, in respect of their proper species, they are more grievous. For unbelief is due to a man not believing God's own truth; while the hatred of God arises from man's will being opposed to God's goodness itself; whereas despair consists in a man ceasing to hope for a share of God's goodness. Hence it is clear that unbelief and hatred of God are against God as He is in Himself, while despair is against Him, according as His good is partaken of by us. Wherefore strictly speaking it is more grievous sin to disbelieve God's truth, or to hate God, than not to hope to receive glory from Him.
If, however, despair be compared to the other two sins from our point of view, then despair is more dangerous, since hope withdraws us from evils and induces us to seek for good things, so that when hope is given up, men rush headlong into sin, and are drawn away from good works. Wherefore a gloss on Proverbs 24:10, "If thou lose hope being weary in the day of distress, thy strength shall be diminished," says: "Nothing is more hateful than despair, for the man that has it loses his constancy both in the every day toils of this life, and, what is worse, in the battle of faith." And Isidore says (De Sum. Bono ii, 14): "To commit a crime is to kill the soul, but to despair is to fall into hell."
Source: Summa Theologica (New Advent)