How do we know that Christ's death was for our sins?

In Isaiah 53, the prophecy emphasizes that 'He shall bear their iniquities' (Isaiah 53:11), revealing that Christ's suffering was specifically for those whom God had designated. This aligns with the idea of substitutionary atonement where Jesus, through His perfect obedience, satisfied the demands of God's justice on behalf of His elect. The certainty of His death as an atonement for sin hinges on God's immutable purpose established before the foundation of the world, showcasing a divine plan where Christ was destined to save His people, thus providing believers with the assurance of their salvation.