One who has become a priest not on the basis of a regulation as to his ancestry but on the basis of the power of an indestructible life. (NIV)
Satan cannot hurt you until your task for Christ is completed. Until that time comes you are truly immortal. He may roar. He may threaten. But he is powerless. When your earthly task is completed (and it may be completed when you are young), it is possible that he may kill you. But you will achieve by your death as much and more than by your life. Satan is powerless to stop the advance of the gospel through your life.
The brethren overcame Satan in three ways. They overcame him as the Accuser by the blood of the Lamb. They overcame him as the deceiving serpent by the word of their testimony. And they overcame him as the devouring dragon because they loved not their lives unto the death. Such people are invincible. Nothing can stop them, and nothing ever will.