Jesus Christ's sacrifice on the cross fulfilled the Law of Moses, so that we would not have to continue trying to do the Law that we could never fulfill. Doing the Law meant that we were always in a position to die in our sins. No matter how hard we tried to be good and do good, that's just the way that it was.
Do you see how unfair that would have been, for us to die in the sin condition that we were born with because we ourselves could never fulfill the requirements to be forgiven of those sins? God is the Good Father and he provided for those unredeemable sins to be redeemed through the sacrifice of Christ Jesus, Our Lord.
