Macrina Walker of A Vow of Conversation has very helpfully  transcribed a talk by Metropolitan Kallistos Ware on "Salvation in Christ".  Ware summarizes the various theological interpretations of just how the life, death, and resurrection of Jesus "save" us.  The excerpt below deals with Jesus as the victor over sin and death.

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Here Christ's work of salvation is seen as a cosmic battle between good and evil, between light and darkness.  Dying on the cross and rising from the dead, Christ is victor over sin, death, and the devil.  this victory is summed up in the last word that he spoke on the cross, tetelestai (Jn 19:30) which is usually translated "It is finished".  But this is not to be seen as a cry of resignation or despair.  Christ is not just saying, "It's all over.  This is the end", but he is affirming, "It is accomplished.  It is fulfilled.  It is completed"...

The Father who particularly uses the idea of victory is St Irenaeus of Lyons at the end of the second century.  If you want to see the idea of victory lived out, then think above all of the Paschal Midnight service, with its constant refrain, Christos anesti ek nekron, "Christ is risen from the dead, trampling down death by death"...

...when Christ says "it is finished", the Evangelist intends us to think back to what was said four chapters earlier, "Having loved his own, he loved them to the end", eis telos.  From this we understand exactly what is finished on the cross, what is fulfilled:  it is the victory of love.  Despite all the suffering, physical and mental, inflicted on him, Jesus goes on loving humankind; his love is not changed into hatred.  We are to see the victory then as not a military victory but as the victory of suffering love, unchanging love, love without limits.  As the Protestant theologian Karl Barth said, "the Christian God is great enough to be humble".  And that's what we see above all in his victory on the cross.  God is never so strong as when he is most weak.