Objection 1. It would seem that presumption is not a sin. For no sin is a reason why man should be heard by God. Yet, through presumption some are heard by God, for it is written (Judith 9:17): "Hear me a poor wretch making supplication to Thee, and presuming of Thy mercy." Therefore presumption on God's mercy is not a sin.
Objection 2. Further, presumption denotes excessive hope. But there cannot be excess of that hope which is in God, since His power and mercy are infinite. Therefore it seems that presumption is not a sin.
Objection 3. Further, that which is a sin does not excuse from sin: for the Master says (Sent. ii, D, 22) that "Adam sinned less, because he sinned in the hope of pardon," which seems to indicate presumption. Therefore presumption is not a sin.
On the contrary, It is reckoned a species of sin against the Holy Ghost.
I answer that, As stated above (Question 20, Article 1) with regard to despair, every appetitive movement that is conformed to a false intellect, is evil in itself and sinful. Now presumption is an appetitive movement, since it denotes an inordinate hope. Moreover it is conformed to a false intellect, just as despair is: for just as it is false that God does not pardon the repentant, or that He does not turn sinners to repentance, so is it false that He grants forgiveness to those who persevere in their sins, and that He gives glory to those who cease from good works: and it is to this estimate that the movement of presumption is conformed.
Consequently presumption is a sin, but less grave than despair, since, on account of His infinite goodness, it is more proper to God to have mercy and to spare, than to punish: for the former becomes God in Himself, the latter becomes Him by reason of our sins.
Reply to Objection 1. Presumption sometimes stands for hope, because even the right hope which we have in God seems to be presumption, if it be measured according to man's estate: yet it is not, if we look at the immensity of the goodness of God.
Reply to Objection 2. Presumption does not denote excessive hope, as though man hoped too much in God; but through man hoping to obtain from God something unbecoming to Him; which is the same as to hope too little in Him, since it implies a depreciation of His power; as stated above (1, ad 1).
Reply to Objection 3. To sin with the intention of persevering in sin and through the hope of being pardoned, is presumptuous, and this does not diminish, but increases sin. To sin, however, with the hope of obtaining pardon some time, and with the intention of refraining from sin and of repenting of it, is not presumptuous, but diminishes sin, because this seems to indicate a will less hardened in sin.
Source: Summa Theologica (New Advent)