Why Did God Allow Satan to Exist?

Q: Why did God allow Satan to exist?

If by “exist”, you mean why is Satan allowed to roam freely on earth, I’d say it is because he has legal rights to do so. And the person who gave him this freedom was Adam when he disobeyed God to accept Satan’s offer. But if by “exist”, you mean why was Satan created, I’d say that just as with everything and everyone God creates, Satan (before he became Satan) was also created for a purpose, and that purpose was for good and not evil. All things were created for God’s glory and pleasure, including Satan. Rev 4:11, Isaiah 43:7.

There’s a spiritual law that says you belong to who you obey. When mankind obeyed Satan in the Garden of Eden, they became Satan’s lawful captives and slaves, and whatever belonged to man could be taken over by Satan, Romans 6:16. 

Remember when Satan tempted Jesus with the kingdoms of the world and their glory? He said he had legal rights to give to whomever he wanted because it was delivered to him, Luke 4:6-7. It was Adam who delivered it to him. If Jesus (the second Adam) had obeyed Satan and bowed down to him, Satan would have given him the whole world, but Jesus would have become his lawful captive according to Romans 6:16, putting Satan in the position of ownership over whatever Jesus had (including what he gave Him). Thank God Jesus Christ was victorious.

Back to the question, Satan exists today to serve the purposes of God mostly in dealing with the flesh, 1 Corinthians 5:5, 2 Corinthians 12:7-9, Jam 1:2-4, Romans 8:28.