Okay here is my two cents and I hope they don't piss a lot of people off(too much)
Jesus was crucified because of the Pax Romana. Christian doctirine was against sacrifices. According to Roman belief sacrifices were neccesary to keep the gods happy so that they would protect Rome(thats what the Pax Romana is).Romans were actually very tolerant of different religions and cultures, so long as they did nothing to threaten the Pax Romana. So here comes Jesus saying that you don't need to sacrifice animals and the like, all you need do is believe in me. As you can see this becomes a threat to the Pax Romana, especially since Jesus was gaining a following. which is why he was crucified as an enemy of Rome.
Now in comes the argument against Jesus dying for our sins.
If a man kills another man, would hanging a third man make the first man any less guilty of murder? I would hope not.
This also brings along the idea that everyone is a sinner in someway or somehow. The main argument for this is the idea of original sin, or that we are concieved in sin, well I for one would hope that I am not being held responsible for others actions, if I am damned to hell I would hope it was for something that I did, not my parents or Adam and Eve. And if the taking of life is a sin, then why is the creating of life a sin? It doesn't make sense.
So for Jesus to have died for our sins we first have to have sinned, and then someone else has to be able to take the responsibility for our sins, which means that I could go out rape, murder, steal, and any other number of sins, then say Jesus died for that so let me in.
And what about babies that die during birth or are stillborn? Do they go to hell because they died before knowing about Jesus?
And why should someone who lives a good life and does wrong to nobody burn in hell because he doesn't believe in Jesus?