Ranking of Sins in Dante's Inferno

It is fascinating to me to view Christianity from Dante's perspective, and to see how the time period in which he lived shaped his religion compared to the time period in which we live today. While Dante is quick to speak a lot of Biblical truth in the poem, like the emphasis that Christians should live their lives giving the glory to God, the ranking of sins doesn't really make sense to me in the context of the Christian principles that Dante is founding his poem on. Of course, caught up in my egocentric life in the 21st century, I can't even fathom the culture back in the 1300s.
The Bible proclaims the truth that Jesus Christ died to forgive sins. Not some sins, not just the ones that were "little", but all sins. No sin is better or worse than the other - everyone is equal at the foot of the cross. Honestly, I think that's one of the coolest parts of Christianity - the fact that even though in our world it seems as if fraud would be worse than incontinence, for example, no sin is beyond the power of Jesus to cleanse.
In Canto XI, Dante says, "...Fraud is man's peculiar vice; God finds it more displeasing - and therefore, the fraudulent are lower, suffering more"(25-27). But what makes this sin worse than lust, sloth, pride? Dante's take on this question stems from intentions - fraud is intentionally trying to gain through someone else's loss while lust is just a consequence of abandoning reason and acting out of haste. To me, although it is hard to accept in our world, the Bible says that sin is sin and whether you accept or reject the grace of Jesus determines whether you will be washed clean of all of your sins, or whether you will experience eternal death. Is that alone not enough punishment? Eternal death?
I personally believe that we are all sinners, and we were born with sin in our hearts. But none of us is worse than the other by what we do, and neither are we better than each other by what we do. In Canto XXXIII, Dante even says that Fra Alberigo was pulled from earth into Hell because of the magnitude of his sins. In my opinion, nothing on this earth can make me greater or less in the eyes of God, reflected by the beliefs of Christians in Dante's time period when they emphasized a life avoiding glorifying themselves. While Dante's levels of Hell and ranking of sins is fascinating, I can't help but wonder what about his religion or teachings that he may have heard led him to this theory that some sinners should be punished worse than others.