It has been taught that when Adam sinned, he gave rulership of the world and all that was in it to the devil. I do not believe this is true. What is true is that Adam's sin went so far as to foul even the heavenly untinsils of worship, because the blood of Jesus was needed to cleanse even these. I also believe that what rules the earth and all that is in it is death. This is the last enemy to be defeated, in all of creation. For even the creation has been subjected to futility.
Satan is a god, obviously a false one, but a god all the same. There are many who worship him. I said earlier that he rules the world, but that is too simple an explanation. The devil rules by deception. The trouble is that without God's redemptive work in our hearts and minds, we are completely susceptible to the devil's deceptions. One who has not received the Holy Spirit has no hope of know the truth about right and wrong, nor about righteousness. And even worse, they are an enemy of God by birth, by their very nature. The prophet wrote that there is none good, not even one. Jesus told the Jews who were arguing with Him that they were of their father, the devil.
God is in control, but He has allowed the devil a certain latitude in his actions, except in the case of His servants. And some of them will be tested until they fall, so that they can be made pure.
In Christ