And then there is the vice of presumption. It is probably more difficult for us to recognize what exactly constitutes presumption. On the one hand, we know that hope is supposed to be certain and sure – how, then, can we have an “excess” of hope? If hope is already 100% certain, how can presumption go beyond the proper boundaries and make us too hopeful?
The truth is that the sin of presumption (like that of despair) can be twofold: We presume either on God or on ourselves. To presume upon ourselves is to believe that we can attain to heaven by our own powers – it is to put our hope in “horse and chariot” (i.e. in our own abilities) rather than in the Name of the Lord (cf. Psalm 19:8, Vulgate). This sort of presumption will usually lead to despair, since it will eventually become clear that our own powers will not save us.
The second form of presumption is the worse, it is to presume upon God’s mercy as being opposed to the divine justice. This is the sin of presumption whereby a man admits that he is a sinner and does not have any right to heaven (nor any means of getting to heaven, by his own powers), but then simply considers the divine mercy and ignores the demands of justice. This is a very prevalent sin in our day. A man who sins by presuming upon God will say something like this: “Sure, I know that I am a sinner, but I am certain that God will save me anyways.”
To think that God will save us and forgive us even if we do not convert, is to commit the sin of presumption. There are some things God cannot do: He cannot make a rock so big he can’t lift it, he cannot forgive an unrepentant sinner, and he cannot make someone who dies in mortal sin to go to heaven.
Do take the time to read the article in its entirety, and also circle back with the CCC to remind ourselves what constitutes a mortal sin.