How do we know that salvation comes through Christ's death?

In John 3:14, Jesus correlates his future crucifixion to the lifting up of the serpent by Moses, indicating that just as the Israelites looked to the serpent for healing, so must people look to Him for salvation. His death was necessary, not a tragic event but a preordained plan for the redemption of His people. Furthermore, His sacrificial death satisfies God's justice, confirming that He paid the penalty for sin once and for all, thus providing eternal life to those who believe in Him.