Key of David, and sceptre of the house of Israel, you open, and no one shuts, you shut, and no one opens: come, and lead the prisoner from jail, seated in darkness and in the shadow of death.
Orthodoxy says that when our first parents sinned, they condemned all of us to death: not out of guilt, mind you, but rather genetics. If you are immortal, your children will be as well. If you are mortal, however, your children will also die. We are in the shadow of death not because of some "stain of original sin", but rather because we are the legitimate children of our parents. More: we die because God's mercy lets us die or else sin would go on forever.
Jesus has trampled down death by death it is death that becomes the gate of mercy by which we escape death, the real death: severance from God caused by sin.
In the shadow of death we all lived in fear: but now, freed, we see death as the gateway of life; as a babe in the womb may think of birth as a terror, but it is only the beginning, so death is for us: scary perhaps, but only the beginning of joy.
Jesus is the Key that unlocks this mystery. It's totally not enough that Jesus was perfect, or that Jesus died "for our sins". Jesus had to die as a human so that humans would never die again.
Yes, of course, we die: the world dies, everything in this world dies. (Genetics, remember?) But death is no longer the end: it is the beginning. "For thy faithful people, O Lord, life is not ended, but changed" as says the burial rite in the new form. We are not taken from life, we are rather moved from life to life, from strength to strength, like sparks among the stubble.
One can be tempted to see this in a "gnostic" light, where this life is not real (like in Plato's cave) and the next life is real. But no, this life is real. Death is real. The next life is more real. We see through a glass darkly now, but we do, in fact, see. The struggles of this world are real, the jail is real, the jailer is real. We are in real danger of death - the real death, severence from God, if we die in our sins.
It's just that our fear is unwarrented.
And Christmas is the end of all that.
All things work for the good of those that love the Lord.
And nothing - nothing - can sever us from the Love of God which is in Christ Jesus our Lord.