Why is Satan's power broken for believers even though he is still alive?
Satan remains alive and active, but his power is broken for believers because Christ's death at the cross defeated sin and utterly defeated Satan. The cross accomplished reconciliation and ended sin's overpowering victory. Believers benefit from that finished work; Satan's hold is broken, not because he is dead or inactive.
Les explains that although Satan is still alive and well, his power has been broken for the benefit of the believer as a result of the work of the cross. In the 490-year prophecy, finishing transgression, making an end of sins, and making reconciliation for iniquity were all accomplished in the work of the cross. Christ died, and that is when reconciliation was made possible. Sin was defeated at the cross, and that is where Satan was utterly defeated. Paul brings out the ministry of reconciliation in 2 Corinthians 5: the lost world has already been reconciled, and it is all been accomplished. Believe it, and God will reckon you as reconciled. So for the believer, Satan's power has been broken because of the finished work of the cross, even though Satan himself is still alive.