I just watched this movie yesterday. One quote left in my mind, "Be careful, Michael. Choosing not to believe in devil won’t protect you from it!”
Michael Kovak (Colin O'Donoghue) passes four years in a seminary school and abdicates his vows upon completion. However, after ordination, he writes a letter of resignation to his superior, Father Matthew, due to a lack of faith. But Father Matthew (Toby Jones) believes that Michael is called to be a priest. He later approaches Michael with an invitation to travel to Rome in order to attend a class on exorcism. Michael reluctantly accepts after being told by Father Matthew that he will be levied a $100,000 student loan if he leaves immediately, but that if he still desires to resign from his position after taking the class, then they will discuss it then (hinting that he may be free to leave).
During classes, he finds his self is skeptic and very tentative in his faith. Father Xavier (Ciarán Hinds) later asks Michael to see a friend of his, Father Lucas (Anthony Hopkins), who is a renowned Welsh Jesuit exorcist. However, Michael remains skeptical, even after witnessing several preternatural events.
The conflict had climax when Father Lucas begins behaving strangely, exhibiting signs of demonic possession. Finally, due to the condition of no one can perform the exorcism except him; Michael decides to perform the exorcism on his own. After constant rebuking by the demon and a long, drawn out fight, Michael regains his once lost faith and is able to force the demon. In the fight, the demon asks Michael to believe in him. Michael says, “Yes, I believe in you. So, I believe in God, too.” He completes the exorcism, and the powerful demon leaves Father Lucas.