Question: “Have you ever seen a demon? What do you think of the Christian’s that say, “oh this specific person has a demon because of x y or z.”
Answer: I personally have not seen a demon but the Bible speaks very clearly about their existence (cf. Mark 5:1-20; Acts 19:13-17). The intent of demonic spirits is to deceive people from the truth which is found in Jesus Christ. There is a possibility for people to teach a false message about Bible doctrines yet not be possessed (cf. I Timothy 4:1-3). I think this may be what you’re referring to about someone “having a demon.” Believers are sealed with the Holy Spirit and no longer their own (cf. Ephesians 1:13, I Corinthians 6:19-20). This means they cannot be possessed by a demon. However, if they do not rightly diving the Word and/or live carnally (that is to say “in their flesh”) then they can be deceived into teaching doctrines from demons. I hope this makes sense.