In For I Have Sinned, the third victim of a serial killer is wearing a silver cross when she is murdered. When Natalie removes it along with the rest of her effects, she suddenly realizes that she has missed noticing that all the victims had been Catholic. She shows the cross to Nick; but he shies away from it. This startles her: she sees no reason why vampires should shun such religious symbols. Nick explains that it is a symbol of the light; and as such, it burns vampires just as the sun does.
Nick takes the cross home with him in an evidence bag. He removes it, carefully; and, dangling it from its chain, he lowers it onto the palm of his other hand.
The cross immediately starts to burn his flesh. After only a moment, he can bear it no longer. When he removes it, a deep red burn mark is left behind in the shape of the cross.
After the case has been resolved and the killer caught, the victim's priest, Father Pierre Rochefort, gives the cross to Nick. He takes it, and finds to his astonishment that it no longer burns him. However, in subsequent episodes, his allergy to crosses remains as powerful as ever.