Objection 1. It would seem that the sin of our first parents was more grievous than other sins. For Augustine says (De Civ. Dei xiv, 15): "Great was the wickedness in sinning, when it was so easy to avoid sin." Now it was very easy for our first parents to avoid sin, because they had nothing within them urging them to sin. Therefore the sin of our first parents was more grievous than other sins.
Objection 2. Further, punishment is proportionate to guilt. Now the sin of our first parents was most severely punished, since by it "death entered into this world," as the Apostle says (Romans 5:12). Therefore that sin was more grievous than other sins.
Objection 3. Further, the first in every genus is seemingly the greatest (Metaph. ii, 4 [Ed. Diel. i, 1). Now the sin of our first parents was the first among sins of men. Therefore it was the greatest.
On the contrary, Origen says [Peri Archon i, 3: "I think that a man who stands on the highest step of perfection cannot fail or fall suddenly: this can happen only by degrees and little by little." Now our first parents were established on the highest and perfect grade. Therefore their first sin was not the greatest of all sins.
I answer that, There is a twofold gravity to be observed in sin. one results from the very species of the sin: thus we say that adultery is a graver sin than simple fornication. The other gravity of sin results from some circumstance of place, person, or time. The former gravity is more essential to sin and is of greater moment: hence a sin is said to be grave in respect of this gravity rather than of the other. Accordingly we must say that the first man's sin was not graver than all other sins of men, as regards the species of the sin. For though pride, of its genus, has a certain pre-eminence over other sins, yet the pride whereby one denies or blasphemes God is greater than the pride whereby one covets God's likeness inordinately, such as the pride of our first parents, as stated (2).
But if we consider the circumstances of the persons who sinned, that sin was most grave on account of the perfection of their state. We must accordingly conclude that this sin was most grievous relatively but not simply.
Reply to Objection 1. This argument considers the gravity of sin as resulting from the person of the sinner.
Reply to Objection 2. The severity of the punishment awarded to that first sin corresponds to the magnitude of the sin, not as regards its species but as regards its being the first sin: because it destroyed the innocence of our original state, and by robbing it of innocence brought disorder upon the whole human nature.
Reply to Objection 3. Where things are directly subordinate, the first must needs be the greatest. Such is not the order among sins, for one follows from another accidentally. And thus it does not follow that the first sin is the greatest.
Source: Summa Theologica (New Advent)