Just to confirm what you said with some Scripture, start with 1 Cor 6:19
"Do you not know that your bodies are temples of the Holy Spirit, who is in you, whom you have received from God? You are not your own;"
Paul makes it very clear that not only are we the temple of the Holy Spirit, but that the Holy Spirit dwells within us. Can a demon possess the Holy Spirit? Of course not? How absurd! As Christians we have the Holy Spirit dwelling in us. We don't need to cast out demons, because God has already finished that, when we were justified!
"What harmony has Christ with Belial, or what has a believer in common with an unbeliever? Or what agreement has the temple of God with idols? For we are the temple of the living God; just as God said, “I will dwell in them and walk among them; and I will be their God, and they shall be My people” (2 Cor 6:15–16).
"There is no clear example in the Bible where a demon ever inhabited or invaded a true believer. Never in the New Testament epistles are believers warned about the possibility of being inhabited by demons. Neither do we see anyone rebuking, binding, or casting demons out of a true believer. The epistles never instruct believers to cast out demons, whether from a believer or unbeliever. Christ and the apostles were the only ones who cast out demons, and in every instance the demon-possessed people were unbelievers."
https://www.gty.org/library/questions/QA191/can-christians-be-demonpossessed
A Christian cannot be possessed! We belong to the kingdom of God, not the Kingdom of the enemy. His demons have no right to possess us!
"and giving joyful thanks to the Father,who has qualified you to share in the inheritance of his holy people in the kingdom of light. 13 For he has rescued us from the dominion of darkness and brought us into the kingdom of the Son he loves, 14 in whom we have redemption, the forgiveness of sins." Col 1:12-14
As for the matching shirts the church members are wearing, sounds like a cult to me!