Matthew 8:31

And the demons besought him, saying, If thou cast us out, send us away into the herd of swine.

This shows that the emissaries of Satan are restricted and may not enter even a herd of swine without the Lord's permission. Other restrictions of Satan are given in 1 Corinthians 10:13 and Matthew 13:25. This request of the demons is a prayer of sorts and indicates that God may indeed answer any prayer that is in harmony with His will, regardless of the wickedness of the petitioner. Thus, he permitted Satan to "sift" Peter. Also, Satan's request with reference to Job was also granted. This is a warning that it is surely a mistake to make the answer to someone's prayer "prima facie" evidence of pardon or forgiveness of sins. How unhappy must be the state of demons which cannot rest except when engaged in tormenting or destroying other beings in God's creation; and, even then, there is no suggestion that they are in any true sense actually happy, except in a relative or accommodative sense.