Objection 1. It would seem that presumption, which is a sin against the Holy Ghost, trusts, not in God, but in our own power. For the lesser the power, the more grievously does he sin who trusts in it too much. But man's power is less than God's. Therefore it is a more grievous sin to presume on human power than to presume on the power of God. Now the sin against the Holy Ghost is most grievous. Therefore presumption, which is reckoned a species of sin against the Holy Ghost, trusts to human rather than to Divine power.
Objection 2. Further, other sins arise from the sin against the Holy Ghost, for this sin is called malice which is a source from which sins arise. Now other sins seem to arise from the presumption whereby man presumes on himself rather than from the presumption whereby he presumes on God, since self-love is the origin of sin, according to Augustine (De Civ. Dei xiv, 28). Therefore it seems that presumption which is a sin against the Holy Ghost, relies chiefly on human power.
Objection 3. Further, sin arises from the inordinate conversion to a mutable good. Now presumption is a sin. Therefore it arises from turning to human power, which is a mutable good, rather than from turning to the power of God, which is an immutable good.
On the contrary, Just as, through despair, a man despises the Divine mercy, on which hope relies, so, through presumption, he despises the Divine justice, which punishes the sinner. Now justice is in God even as mercy is. Therefore, just as despair consists in aversion from God, so presumption consists in inordinate conversion to Him.
I answer that, Presumption seems to imply immoderate hope. Now the object of hope is an arduous possible good: and a thing is possible to a man in two ways: first by his own power; secondly, by the power of God alone. With regard to either hope there may be presumption owing to lack of moderation. As to the hope whereby a man relies on his own power, there is presumption if he tends to a good as though it were possible to him, whereas it surpasses his powers, according to Judith 6:15: "Thou humblest them that presume of themselves." This presumption is contrary to the virtue of magnanimity which holds to the mean in this kind of hope.
But as to the hope whereby a man relies on the power of God, there may be presumption through immoderation, in the fact that a man tends to some good as though it were possible by the power and mercy of God, whereas it is not possible, for instance, if a man hope to obtain pardon without repenting, or glory without merits. This presumption is, properly, the sin against the Holy Ghost, because, to wit, by presuming thus a man removes or despises the assistance of the Holy Spirit, whereby he is withdrawn from sin.
Reply to Objection 1. As stated above (20, 3; I-II, 73, 3) a sin which is against God is, in its genus, graver than other sins. Hence presumption whereby a man relies on God inordinately, is a more grievous sin than the presumption of trusting in one's own power, since to rely on the Divine power for obtaining what is unbecoming to God, is to depreciate the Divine power, and it is evident that it is a graver sin to detract from the Divine power than to exaggerate one's own.
Reply to Objection 2. The presumption whereby a man presumes inordinately on God, includes self-love, whereby he loves his own good inordinately. For when we desire a thing very much, we think we can easily procure it through others, even though we cannot.
Reply to Objection 3. Presumption on God's mercy implies both conversion to a mutable good, in so far as it arises from an inordinate desire of one's own good, and aversion from the immutable good, in as much as it ascribes to the Divine power that which is unbecoming to it, for thus man turns away from God's power.
Source: Summa Theologica (New Advent)