When we choose not to obey God's revealed will to us, this is disobedience, and this removes His protection over us and our minds become open to Satan's control. In James 4:7 it states "Submit therefore to God, resist the devil, and he will flee from you." We have no power over Satan. The only power we have over Satan is Jesus' power, and His power is only available to us when we are walking in obedience to Him. When we are not choosing to walk in obedience to Christ, we cannot resist the devil and we become open and under His control, and Satan gains control over our minds. James 4:7 tells us that as long as we are walking in obedience to God in our use of time we can resist the Devil and that he will flee, and as long as we are focusing with our minds on God's will and doing God's will with our use of time, we will have control of our minds. Satan can only gain access to the mind of a disobedient Christian. Another way Satan gains control of our minds is when we deliberately choose to sin. God never takes control of our minds. He created us in His image. That means like God we have free independent will. God never violates the independent will of anyone. Even after we give our lives to Christ, he does not take control of our minds. Jesus will tell us what His will for our lives are, but he will never force us to do his will. We are always in control of ourselves and must choose to obey Him. He never requires us to yield our minds and bodies to Him to control.
Satan was the beginning of evil. He is a spirit being. When adam & eve sinned ; we inherited the sin nature. Each one of us decides to follow Jesus or Satan. As a spirit satan can be anywhere. He also took 1/3 of the evil angels with him. So he has many many helpers.