There are several ATONEMENT THEORIES.... all quite ancient, all quite biblical, all seemingly falling short.
This we know: Atonement/Justification required Jesus, it required the Incarnation of God, it required His perfect life/absolute righteousness, it required His sacrificial death, and (it seems above all) His victorious resurrection. THAT seems clear!
But WHY? That's where it gets a bit... well.... undisclosed. There arose several different paradigms, several different ways of putting all this, and these were all highly developed by the early Middle Ages. Augustine, Luther, Calvin all seemed to like the Christus Victor view.... modern American "Evangelicals" seem to like the "vicarious" view... there are several more. All seem great - as far as they go, but all seem to have limitations.
My take: Jesus' incarnation, life, death and resurrection were ALL completely, absolutely necessary! WHY? Um.... there's some mystery there. But THIS IS THE POINT: The necessary was provided and fulfilled - completely, perfectly - BY JESUS. That's the point. That's the Gospel.
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