Fright Night

Seventeen-year-old Charley Brewster is a fan of both traditional horror films and a horror TV series entitled Fright Night, hosted by former movie vampire hunter Peter Vincent. One evening, Charley discovers that his new next door neighbour Jerry Dandrige, is a vampire responsible for the disappearences of several victims. Charley tries to tell his mother and asks his friends for help. In desperation, he contacts the police, but they believe that he is imagining things and ignore his claims after revealing his suspicions to them when he accompanies a homicide officer to Jerry's house. That night, Charley gets a visit from Jerry, who offers Charley a "choice" by forgetting about his vampire identity, but he refuses by using his crucifix on Jerry. When Jerry stops Charley and slowly tries to push him out the window to his death, he stabs Jerry's hand with a pencil. Enraged, Jerry destroys Charley's car in retaliation and threatens Charley that he plans to do much worse to him later.
Charley turns to Peter Vincent for help, but Peter dismisses Charley as an obsessed fan. Charley's girlfriend, Amy Peterson, fears for Charley's sanity and safety, resulting her hiring the destitute Vincent to "prove" that Jerry is not a vampire by having him drink what they claim is "holy water", but it turns out to only be tap water; Jerry having claimed to Peter that drinking actual holy water would be against his religious convictions. Vincent discovers Jerry's true nature after glancing at his pocket mirror and noticing Jerry's lack of a reflection, causing him to accidentally drop and smash the mirror. Vincent then flees, but Jerry learns of his discovery after finding a piece of his pocket mirror on the floor.
Jerry hunts down and turns Charley's friend, "Evil" Ed Thompson, into a vampire. Evil then visits Vincent and tries to attack him, only to be warded off when injured by a crucifix. Meanwhile, Jerry chases Charley and Amy into a club. While Charley is trying to call the police for help, Jerry hypnotizes and abducts Amy who bears a resemblance to Jerry's lost love (whom Jerry has a painting of). With nowhere left to turn, Charley attempts to gain Vincent's help once more. A frightened Vincent (following Evil's attack) initially refuses, but he then reluctantly resumes his "Vampire Killer" role as Charley approaches his neighbour's house. The two are able to repel Jerry's attack using a crucifix, though only Charley's works since he has faith in its spiritual power. Jerry's protector, Billy Cole, appears and knocks Charley unconscious over the banister, having Vincent flee to Charley's house. At his house, he finds that Mrs. Brewster is still not home and is attacked by Evil, who transforms into a wolf. Vincent seemingly kills him after staking him through the heart but later removes the stake. Meanwhile, an unconscious Charley is taken to Amy who has been turned into a vampire by Jerry. Vincent says the process can be reversed, but only if they kill Jerry before dawn.
Charley and Vincent are then confronted by Billy whom Vincent shoots since he saw his reflection in the mirror. However, Billy is revealed to be a zombie-like creature, who rises again and continues advancing towards them until he is killed by Charley who stakes him and then melts into goo and dust. Jerry appears, but Vincent is able to lure the overconfident vampire in front of a window using a crucifix (now working due to his renewed faith in its abilities). Just before the morning sun, Jerry transforms into a bat and attacks Vincent and Charley (biting Charley in the process) before fleeing to his coffin in the basement. Charley and Vincent go in pursuit of Jerry; Vincent breaks open Jerry's coffin and tries to stake him through the heart whilst Charley has to fight off Amy who has completed her transformation. By breaking the blacked-out windows in the basement, Vincent and Charley expose Jerry to the sunlight and kills him. Jerry's death leads Amy reverting to her human form and the three embrace.
A few nights later, Vincent returns to his Fright Night TV series and announces a hiatus from vampires by instead presenting Octaman. The series is being watched by Charley and Amy as they embrace together in bed. When Charley goes to turn off the TV, he at first sees red eyes in Jerry's now-vacant house, but dismisses them. Unbeknownst to both Charley and Amy, a survived Evil with red eyes (hiding in the darkness) laughs and says "Oh, you're so cool, Brewster!"

Question: For what reason did Jerry claim he wouldn't drink actual holy water?
Answer: against his religious convictions