The devil has real power, this is confirmed by God himself.
Job 1:12 And the LORD said unto Satan, Behold, all that he hath is in thy power; only upon himself put not forth thine hand. So Satan went forth from the presence of the LORD.
Lots of folks think that Satan has to get permission from God to hurt anyone. This is not true, God has only promised to protect those who obey him and are "in all thy ways" (Psa 91:11). This is the part of scripture Satan left out when he tempted Jesus in the wilderness. For those who are not obedient to God, there is no promise of protection and Satan can harm them at his will.
2 Tim 2:26 And that they may recover themselves out of the snare of the devil, who are taken captive by him at his will.
That not all people have this protection is shown by the conversation Satan had with God in Job.
Job 1:9 Then Satan answered the LORD, and said, Doth Job fear God for nought?
10 Hast not thou made an hedge about him, and about his house, and about all that he hath on every side? thou hast blessed the work of his hands, and his substance is increased in the land.
11 But put forth thine hand now, and touch all that he hath, and he will curse thee to thy face.
Satan recognized that Job was not like other men, but that God had put a hedge about him, his house, and all that he had. This is because Job was very obedient to God. This protection is not promised to the disobedient.
