Unlike the Jesus in most movies, the real Jesus did not whisper, "It is finished." He spoke with victory in His voice. He shouted out, "It- is- finished!" The work the Father had sent Him to do was done. He was the light of God's love, our example of how to live, and He had sacrificed Himself for us all--God laid on Him the sins/iniquities of us all. God accepted His sacrifice and the sins of all who believed in Jesus, and of all who would believe in Jesus because of their testimony, were forgiven. (Isaiah 53) No more would we be slaves to sin or fear death. Jesus knew He would be resurrected and that all who believed in Him would be resurrected as well.
When the centurion saw the way He died, heard the victory cry, he knew without a doubt who Jesus really was. Glory!