Anyone who is a Christian has been taught that Jesus was killed for us and that His death washed our sins away. This is one of a number of essential things to believe to become a Christian.
In his book Mere Christianity, C.S. Lewis asked what possible point could there be in punishing one innocent person instead of the guilty people. And when you think about it from the standpoint of a court system – there is really no point to it at all. But (Lewis said) when you think of it in terms of there being a debt, it makes perfect sense.
There is a point in someone who has plenty of assets paying on behalf of someone who has nothing, or footing the bill for those who are bankrupt.
In our sins, we dug a hole for ourselves. The OT law was given. Man tried to climb out of the hole, using the law. The only thing is, in trying to use the law, we dug the hole even deeper! It only made matters worse. Why? Because the law was bad? Of course not (Romans 7:7). The problem is with us. Paul said (Rom. 7:14), “We know that the law is spiritual; but I am unspiritual, sold as a slave to sin.”
So the law showed us our need for a Savior. He was the only One who had wealth enough to pay our debt – He paid for it out of His righteousness. Paul said that God made Him who “had no sin, to become sin for us, so that in Him we might become the righteousness of God.” (2 Cor. 5:21)