Is it a sin against hope to count upon the possession of God one day, and to hold that such is possible without taking the trouble to prepare ourselves by a life of supernatural virtue?
Yes, this is a sin against hope.
What is this sin called?
It is called presumption (XXI.).
Is this the only sin that can be committed against hope?
No, there is another that is called despair (XX.).
What is the sin of despair?
The sin of despair consists in this: by reason of the high excellence of God, who is to be won as He is in Himself, or by reason of the difficulties of practising supernaturally a virtuous life, one does God the injury to think it impossible to practise this life of virtue and so gain happiness; a virtuous life is therefore renounced and no longer does one seek God's help (XX. 1, 2).
Is the sin of despair a very grave sin?
In some sense it is the gravest sin of all, for of itself it makes all supernatural effort impossible, and the sinner in some sort damns himself (XX. 3).
No matter then how great are man`s miseries and his sins he should never despair?
No, man ought never to despair no matter how great his sins; for the mercy of God is so great and He is so good that He will always help him by His grace.
What then should man do when he feels the weight of his troubles and of his sins?
He should turn to God immediately corresponding to the grace God always gives, and trust that God will give him strength to live a virtuous life, and so gain heaven.
Next - The Catechism of the Summa -SECOND SECTION - A DETAILED SURVEY OF MAN'S RETURN TO GOD - V. OF HOPE AND ITS NATURE -- OF THE VICES OPPOSED THERETO, NAMELY, OF PRESUMPTION AND OF DESPAIR -- OF THE FORMULA OF AN ACT OF HOPE, AND OF THOSE ABLE TO MAKE THIS ACT (D)