When Jesus said it is finished, he had overcome satan and the world.
But he said before he went on the cross that he had finished the works that the father had given him to do, and that was to bare witness to the truth and preach the gospel, and show us a new and living way. And the works that God gave Jesus to do, were the works that Jesus did in his lifetime.
John 5
But I have greater witness than that of John: for the works which the Father hath given me to finish, the same works that I do, bear witness of me, that the Father hath sent me.
So you're saying, "Christ didn't come to earth to die for the sins of mankind?" That would be a pretty bold pronouncement on your part.