The power of the enemy (Satan and the demons, and by extension those who are under their influence) is death: they can kill the body through sin; this is what happened in the Garden and is the reason why our Lord calls Satan the "first murderer" (instead of Cain). However, since Christ voluntarily took death onto Himself (instead of being under its dominion through sin), He was able to show who He truly was by destroying that power through resurrection. We share in that death through baptism, as St Paul says in Romans: his means death has no power over us and, if this is the case, then neither does sin, since "sin is the stinger of death" (1 Cor. 15:56). However, since we have only the first fruits of Christ's life, the Holy Spirit who is God, and God is not yet "all in all" (God has defeated death on the Cross, but defeat in war doesn't mean pacification: the last enemy to be destroyed is death), we can -- voluntarily -- come under the power and dominion of death once again through sin.
How shall we escape this body of death? The Lord renews us in our baptismal death, in our sharing of His Cross, through repentance. In other words, when we die to self in baptism or repentance, we defeat again in Christ the power of the enemy. Death can have no claim on us who repent, but rather God "will shortly trample Satan under your feet" (Rom. 16:20).
Repentance is our freedom from the oppression of death, of demons, of sin; let us join again today in the death of Christ.