“I am He who lives, and was dead, and behold, I am alive forevermore. Amen. And I have the keys of Hades and of Death.”
When the apostle John saw the glorified Jesus Christ in vision shining like the sun in its strength, he “fell at His feet as dead.” But Christ told John, “Do not be afraid” (Revelation 1:17).
Jesus Christ explained that He is eternal. He has always existed in the past and will be alive forevermore. But this eternal being was willing to come to earth as a human being and give His life for our sins! He experienced death, but the Father resurrected Him from the dead and returned Him to His eternal glory and authority.
Jesus encouraged the elderly apostle John by telling him that He who was dead and was resurrected has power over death and the grave. Jesus Christ has the power to resurrect us from death.
“The key here signifies the power and authority over life, death, and the grave. … We should understand hades here, not as hell, nor the place of separate spirits, but merely as the ‘grave’; and the key we find to be merely the emblem of power and authority. Christ can both save and destroy, can kill and make alive. Death is still under His dominion, and He can recall the dead whensoever He pleases. He is ‘the resurrection, and the life’” (Adam Clarke’s Commentary on the Whole Bible, comment on Revelation 1:18).
For more about Christ’s power over death, see “Resurrections: What Are They?”