When a priest performs an exorcism, he is
A) an ordained priest of God, united to Christ in a special way by means of the sacred character imprinted on his soul when he was ordained.
B) doing the job under Obedience to his bishop, which is the best way to stay humble
C) acting with the full power of the Catholic Church behind him, speaking in the Church's name
If a layman were to attempt an exorcism, he would be in QUITE A DIFFERENT SITUATION.
A) A mere man, facing down an angelic intellect with angelic powers.
B) Undertaking the role of "exorcist" out of pride and vainglory. How can one vanquish the devil with pride? That is like fighting a fire using glowing hot coals!
C) He would be acting completely alone, speaking in his own name, using his own personal "power" (if you can call it that!)
I leave you with the words of Holy Scripture, teaching us what can easily happen when self-appointed exorcists, not sent by the Church, attempt to fight the devil:
Acts of the Apostles, chapter 19:
[12] So that even there were brought from his body to the sick, handkerchiefs and aprons, and the diseases departed from them, and the wicked spirits went out of them. [13] Now some also of the Jєωιѕн exorcists who went about, attempted to invoke over them that had evil spirits, the name of the Lord Jesus, saying: I conjure you by Jesus, whom Paul preacheth. [14] And there were certain men, seven sons of Sceva, a Jєω, a chief priest, that did this. [15] But the wicked spirit, answering, said to them: Jesus I know, and Paul I know; but who are you?
[16] And the man in whom the wicked spirit was, leaping upon them, and mastering them both, prevailed against them, so that they fled out of that house naked and wounded. [17] And this became known to all the Jєωs and the Gentiles that dwelt at Ephesus; and fear fell on them all, and the name of the Lord Jesus was magnified.
Wow! Scripture puts it quite delicately here, but the meaning is clear: the possessed man BRUTALLY SODOMIZED the would-be exorcists!
If you think about it, it wouldn't make sense any other way. If you took a toy gun and went after a group of bank robbers, and shouted, "Freeze, dirt bags!" and then started "firing" your toy cap gun, what do you suppose those criminals would do to you, once they realized you were powerless? They would probably kill you, if they were armed.