Supposedly all are 'sinners' because 'sin' is all things that separate one from God. (not just your basic obvious bad deeds). As humans we spend much of our time in modes of thought or behavior that are separating us. (In fact most spiritual pursuits are seeking ways to close that separation.)
SO then JC comes along and is both fully human and at the same time god incarnate. So god gets to see what it is like to walk around and live as one of us. hold that thought for the next part...
So the old testament understanding of god was that 'he' wants you to pay for not following the rules - a sacrifice is demanded. Somehow sacrificing animals was supposed to fill that atonement need. This was the understanding at the time.
So someone/thing has to pay for our sins in that mindset.
Now JC tries to teach it is not about laws etc to be right with god, but what really strikes the point is that he (as man and god - cuz he is one with god, remember) is going to do the needed sacrifice, but the big deal of it is - (remember the 'why has thou forsaken me' part?) he is separated from god at that point - he feels what it is like to be just man separated from god -thus feeling and taking on sin (remember my earlier definition of sin).
SO basically - the thing i get from it is that god loves humans enough to experience the pain of being human, and therefore, we should take confidence that god/the universal force, whatever you want to call it can understand our pain and struggles firsthand - we belong to the universe.
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