And why should we suffer for something our ancestors did?
There are at least two answers to that question. The first is that our ancestors are good representatives for us (what that means is that if we were in their shoes we would have done the same thing they did).
This of course assumes (and it is a very big
assumption) that God even exists at all, much less the Judeo-Christian god specifically. This explanation, of course, assumes that God is omnipotent and knows the future. The problem with that is, if he knew that we would do the same thing if we were in their shoes, he also knew that the first man and woman would sin in the first place.
Christian doctrine has 90% of the human race going to Hell, essentially doomed before their birth, a situation the proposed Judeo-Christian God could have stopped before it started yet chose not to. What that means is he must have thought it was "good" to create billions of humans, knowing that they would be automatically consigned to suffer if he did so. Surely you can see how that is problematic from a philosophical and even a common sense standpoint.
The second is that it isn't our decision to make - God is the final judge and if he chooses to condemn the race for the actions of their parents, there is no higher we authority we can appeal to overturn the decision.
But if said God created us in his own image, the same things that are morally repugnant to us should be morally repugnant to him as well. How many civilized societies do you know of that punish children for the actions of their parents, grandparents, great-grandparents, etc.? Something about the concept seems to go against human nature itself.
In reference to this second answer, this is the answer in my experience we have the hardest time accepting. The first answer is enough to prove that God wasn't acting arbitrarily. In addition to this, he has proven his justice and his good intentions by providing a solution to our sufferings - in fact, he has gone as far as he possibly can to prove it, which is that he, the creator, humiliated himself by taking on human flesh and suffered the penalty of his own law, ie, he suffered death.
The Son who Christianity has as being "crucified before the foundation of the world" still does not soften the idea of a god who knew exactly how every event in history would pan out before it happened and then went ahead and allow it all by creating mankind anyway. The only conclusion one can reach from that is that God actually wanted
billions of humans to go to Hell.
If God knew yet could not choose any other course of action, then said God does not have free will and therefore is not omnipotent. If God could choose and did not, then said God is not all-merciful and all-loving.
So basically what the bible teaches is this. Our parents sinned, and we have to suffer for it, but this fair because if we were in their shoes we would have done the same thing. God proves his goodness by suffering the consequences of sin for those who trust him. This effectively means that Jesus suffered for the sins of others so that they would have hope that there sufferings will someday cease, and that they would have hope of righteousness.
Again how is it fair to consign someone to punishment based on what they "would have done." There is just no way around the conundrum here. Even when I was a Christian, I pushed this issue under the rug and tried not to think about it. Any way you look at it, you have a God who could have prevented the fall of mankind by not creating mankind in the first place yet went ahead and created mankind, thereby proving that it was his will, that he thought that it was "good" for 90% of the human race to be consigned to Hell. No Bandaid one might put on it can change that, whether it be the free will Bandaid, the Savior Bandaid, or any other doctrinal Bandaid one can come up with.
So if we really want to boil things down, it boils down like this. Humanity's greatest need is righteousness. God provides it for us through Jesus Christ. Those who don't believe prove they don't see righteousness as their greatest need and so God doesn't give it to them - they will suffer the consequences of their sin. Those who do believe prove they see righteousness as their greatest need and God gives it to them as a free gift - the sufferings that they experience are nothing compared to the hope they have for the future, and are therefore comforted greatly.
What it boils down to is that our very birth immediately consigns most of us to the most unimaginable torture... and to suffer it forever and ever and ever.
Now as far as the Savior alternative goes, Christian tradition has Jesus suffering for what, a little less than 48 hours as a substitute for eternal punishment? And if we reject Jesus, we still suffer punishment for the crimes he has supposedly already paid for (which is double jeopardy)? And the punishment lasts forever for only a short lifetime of sins? That isn't fair, because the punishment doesn't fit the crime.