You do need faith to cast out demons. If unbelievers try to cast out evil spirits, sometimes it can backfire as in the case below:
Then some itinerant Jewish exorcists tried to invoke the name of the Lord Jesus over those with evil spirits, saying, “I adjure you by the Jesus whom Paul preaches.” When the seven sons of Sceva, a Jewish high priest, tried to do this, the evil spirit said to them in reply, “Jesus I recognize, Paul I know, but who are you?” The person with the evil spirit then sprang at them and subdued them all. He so overpowered them that they fled naked and wounded from that house. (Acts 19:13-16)
The whole world is under the control of the Devil, but God has given Christians the authority to overcome demons as long as they are free of any grounds for demons to access them:
They conquered him by the blood of the Lamb and by the word of their testimony (Revelation 12:11)
Demons need legal rights to possess a person and unbelievers usually are not able to revoke all these legal rights once they have invited evil spirit into their life. So if you have not yet accepted Jesus Christ and become a born-again Christian with the Holy Spirit in you, there is always a chance that they will return even if they leave temporarily.
Legal Rights of Demons - How to Deal With Them
Are you currently going to any church that you can seek help from? It would be good if you could talk to the pastor there.