Even the faithful of ancient Israel knew that they would one day die. They knew it without a doubt, and had no hope that it could be avoided. Delayed, yes; avoided, no.
Death was for them a time of silence, sleep, darkness. It took place in a region below called Sheol. Sheol housed all the dead. It was assumed that those who lived rightously enjoyed a better state than those who lived unrighteously. Yet everyone was dead, and captive to that overall state.
When Jesus was raised from the dead and took all the captives to heaven to be with Him forever, it created a whole new way of looking at life and death. It would take even believing humanity some time to adjust to so dramatic a change.
According to the New Testament, Jesus went to heaven immediately but the rest of the dead did not follow until approximately a generation later. That was when all the messianic promises were fulfilled: the kingdom of God came, the day of the Lord dawned, the new covenant took effect, and the new heavens and new earth were established. This new order of creation no longer had a need for Sheol (Hades) because the dead were all transferred to heaven (“He led captivity captive”) in the instant of transition. Every person who died thereafter would no longer descend to Sheol but rather ascend straight to heaven. There would no longer be a sleep of death. Humans would simply undergo an immediate metamorphosis to the new body that would required for life in heaven.
Because of this new order for us, Jesus could say that we would “live even if we died” and that we would “never die.” The faithful of ancient Israel would understand exactly what He was talking about, for the arrangements for afterlife were different enough to justify such a dramatic description.
Praise be to God who has delivered us from so great a peril as death – it no longer holds any power over us!