Objection 1. It seems that presumption is not a sin. For the Apostle says: "Forgetting the things that are behind, I stretch forth [Vulgate: 'and stretching forth'] myself to those that are before." But it seems to savor of presumption that one should tend to what is above oneself. Therefore presumption is not a sin.
Objection 2. Further, the Philosopher says (Ethic. i, 7) "we should not listen to those who would persuade us to relish human things because we are men, or mortal things because we are mortal, but we should relish those that make us immortal": and (Metaph. i) "that man should pursue divine things as far as possible." Now divine and immortal things are seemingly far above man. Since then presumption consists essentially in tending to what is above oneself, it seems that presumption is something praiseworthy, rather than a sin.
Objection 3. Further, the Apostle says (2 Corinthians 3:5): "Not that we are sufficient to think anything of ourselves, as of ourselves." If then presumption, by which one strives at that for which one is not sufficient, be a sin, it seems that man cannot lawfully even think of anything good: which is absurd. Therefore presumption is not a sin.
On the contrary, It is written (Sirach 37:3): "O wicked presumption, whence camest thou?" and a gloss answers: "From a creature's evil will." Now all that comes of the root of an evil will is a sin. Therefore presumption is a sin.
I answer that, Since whatever is according to nature, is ordered by the Divine Reason, which human reason ought to imitate, whatever is done in accordance with human reason in opposition to the order established in general throughout natural things is vicious and sinful. Now it is established throughout all natural things, that every action is commensurate with the power of the agent, nor does any natural agent strive to do what exceeds its ability. Hence it is vicious and sinful, as being contrary to the natural order, that any one should assume to do what is above his power: and this is what is meant by presumption, as its very name shows. Wherefore it is evident that presumption is a sin.
Reply to Objection 1. Nothing hinders that which is above the active power of a natural thing, and yet not above the passive power of that same thing: thus the air is possessed of a passive power by reason of which it can be so changed as to obtain the action and movement of fire, which surpass the active power of air. Thus too it would be sinful and presumptuous for a man while in a state of imperfect virtue to attempt the immediate accomplishment of what belongs to perfect virtue. But it is not presumptuous or sinful for a man to endeavor to advance towards perfect virtue. On this way the Apostle stretched himself forth to the things that were before him, namely continually advancing forward.
Reply to Objection 2. Divine and immortal things surpass man according to the order of nature. Yet man is possessed of a natural power, namely the intellect, whereby he can be united to immortal and Divine things. On this respect the Philosopher says that "man ought to pursue immortal and divine things," not that he should do what it becomes God to do, but that he should be united to Him in intellect and will.
Reply to Objection 3. As the Philosopher says (Ethic. iii, 3), "what we can do by the help of others we can do by ourselves in a sense." Hence since we can think and do good by the help of God, this is not altogether above our ability. Hence it is not presumptuous for a man to attempt the accomplishment of a virtuous deed: but it would be presumptuous if one were to make the attempt without confidence in God's assistance.
Source: Summa Theologica (New Advent)