Concerning this thing I pleaded with the Lord three times that it might depart from me. And He said to me, "My grace is sufficient for you, for My strength is made perfect in weakness." Therefore most gladly I will rather boast in my infirmities, that the power of Christ may rest upon me.—2 Corinthians 12:8-9
When you put your faith in Jesus Christ, a sign is effectively hung around your neck that says, "Under new management." You now belong to Jesus Christ, and He does not operate on a timeshare program. Isn't that nice to know? He doesn't say, "Okay, now I have Greg for six months, and the devil can have him for the next six months." That is not what happens. When we put our faith in Christ, He comes in and is the sole resident in our hearts and lives.
Although a Christian cannot be demon-possessed, the devil can affect a Christian outwardly. For instance, demons can tempt and oppress a Christian. The apostle Paul wrote, "And lest I should be exalted above measure by the abundance of the revelations, a thorn in the flesh was given to me, a messenger of Satan to buffet me, lest I be exalted above measure" (2 Corinthians 12:7). The word "buffet" means to hit in the face. So Paul was essentially saying, "Yes, I have come under some demon attack. But here is the good news: God will never give you more than you can handle."
The Bible tells us in James 2:19, "You believe that there is one God. You do well. Even the demons believe—and tremble!" Of course, just because you believe something is true doesn't mean you have committed yourself to it. Obviously, demons are in rebellion against God.
Paul's oppression was allowed by God and orchestrated by Satan. So God may allow the devil to tempt you or harass you in some way. But remember, He won't give you more than you can take. The only thing that will stop the devil is the power of Jesus Christ. He is our only protection. He is the one we need.