There's a clear difference between being completely controlled by a demon, which is called demon possession, and just being harassed or tempted by one, which is known as demon oppression or influence. In the Bible, people are only told to resist demons, not to cast them out of believers.
Christians are filled with the Holy Spirit, who wouldn't allow a demon to fully control a Christian. It's impossible to think that God would let a demon possess a Christian, someone He has saved and transformed. The Bible teaches that Christians fight against Satan and demons from the outside, not within themselves, because the Holy Spirit inside them is stronger than any demon in the world.
Despite this, some Bible teachers talk about "demonization" as a way a demon might control a Christian. But, this idea doesn't hold up because it's basically the same as saying a Christian is possessed by a demon, which the Bible says can't happen. Christians can definitely feel the pressure and temptation from demons, but they can't be possessed or fully controlled by them.
We should be careful not to let our own experiences change what the Bible says about demons. If we see someone who seems Christian but acts like they're under a demon's control, we should reconsider if their faith was genuine instead of changing our beliefs about demon possession.
That person might just be heavily oppressed by demons or dealing with big psychological issues, but we should always check these experiences against what the Bible says.