How do we know that Christ's atoning death is necessary for salvation?

The necessity of Christ's atoning death is central to the Gospel. In 1 Peter 3:18, we see that Christ suffered for sins, the just for the unjust, to bring us to God. This illustrates the deep theological truth that God demands justice for sin, and that justice can only be satisfied through the sacrificial death of Jesus. Without His vicarious suffering and death, there can be no payment for sin, and thus no salvation for humanity. His death is the fulfillment of God's promise to redeem a people for Himself, demonstrating both His love and the seriousness of sin.