How does Christ's atonement guarantee my salvation?

The doctrine of atonement underscores that Christ's sacrifice definitively secured the salvation of the elect. In 1 Peter 1:18-19, it states that believers are redeemed with Christ's precious blood, which was foreordained for this specific purpose. If Christ's sacrifice was intended for all humanity and yet some remain lost, this would imply that His blood did not achieve its intended effect, which contradicts the Scriptures. Hebrews 9:12 affirms that Jesus entered the holy place, having obtained eternal redemption for us, solidifying the factual basis of salvation through His completed work. Therefore, the assurance of salvation is rooted in the completed and effective work of Christ on behalf of those whom the Father has given to Him.