Those who committed fraud or malice are the worst because they misused their intellect. Violence or mad bestiality is placed second because those sinners didn't even use their intellect. Finally, those in the incontinence category submitted passions to intellect. Sinners that are guilty of incontinence excessively indulged or submitted to passions, which includes those that were in life lustful, gluttonous, avaricious and wrathful. Each of these sins involves succumbing to non-rational desires, be they spirited or appetitive.
Religion designates degrees of severity to sins.
However, it does not seem that religion is dictating the order of the sins in Dante's Hell. For, religion does not stress the importance of using ones intellect fully and correctly. Religion often stifles freethinking. It forces its followers to think and act in a very strictly contrived fashion without ever questioning the church's teachings. The oppressive nature of religion is clearly exhibited in The Name of the Rose.