death and hell defeated

When Scripture says Satan is defeated and Christ now holds the keys to death and hell, what does this mean? 

This is simply saying that death no longer holds the same meaning and effect it once did. Satan and death no longer have the final word regarding our eternal destination i.e. our eternal separation from God and loss of all the gifts of life we now enjoy. When we are in Christ, death now means we are ushered into his presence when we die and exit this veil of tears. This is why Paul said,

When the perishable puts on the imperishable, and the mortal puts on immortality, then shall come to pass the saying that is written:“Death is swallowed up in victory.”  “O death, where is your victory? O death, where is your sting?” The sting of death is sin, and the power of sin is the law. But thanks be to God, who gives us the victory through our Lord Jesus Christ. 1Co 15:54-57

This does not mean pain and suffering (or Satan) are eliminated in this life, or that death no longer exists (people die every day). It simply means if we are in Christ, death no longer signifies judgment and our ultimate separation from our Creator - nor do the accusations of Satan that we are worthless, guilty, and justly condemned. Why? Because our acceptance by God is based solely on Christ's efforts, not ours. 

Pain, suffering, and death have been conquered, neutered, defused, altered, and are even redeemed and used to bring life in this present world (i.e. to draw us closer to God, who is life). Ultimately, they will no longer be useful or necessary and will eventually be done away with altogether. 

If we are in Christ, they are no longer tools or instruments of destruction and eternal separation from God but are now used as a means of deliverance from our focus and preoccupation with self.

Death no longer means final and eternal death because the sting of death has been removed. Before Christ the sting was that it ultimately led to condemnation and eternal separation from the Source of life, love, and all the blessings of creation. It no longer does for those who are in Christ. Now death is used as a means to life and ushers us into His presence which is fullness of life indeed awaiting us. 

" 'O death, where is your victory? O death, where is your sting?' " The sting of death is sin, and the power of sin is the law. But thanks be to God, who gives us the victory through our Lord Jesus Christ."  1Co 15:55-57

Christ fulfilled the demands of the law and our just condemnation for all the harm we caused others by ignoring it!

2Co 12:7  So to keep me from becoming conceited because of the surpassing greatness of the revelations, a thorn was given me in the flesh, a messenger of Satan to harass me, to keep me from becoming conceited (who do you think gave the thorn...it wasn't Satan himself. And note it is described as a gift, not a curse or punishment).

When the Bible says that Christ now holds the keys to death and hell it does not mean he's eliminated these but by his resurrection he overcame them, and now he's redeeming and using them for our ultimate good and his glory. All this is offered to us if we receive His gift of eternal life in, by, and through Christ! Only through Christ is this true.