Luke 10:20 (KJV) Notwithstanding in this rejoice not, that the spirits are subject unto you; but rather rejoice, because your names are written in heaven.
If you are saved, you have your name written in heaven. Many times, saved people rejoice when they overcome evil. Jesus said we should rejoice because we are saved and not because we can overcome evil. Why is this? If others see our rejoicing when we overcome evil, they may not know that we can overcome evil because we are saved. Others naturally will want to overcome evil too – but if they see us rejoicing that we are saved, they will learn how to overcome evil AND learn how to be saved as well.