In Chapter 13 of Dracula, after Lucy Westenra has died and they are making funeral plans, Prof. Van Helsing mentions to Jack Seward that he plans to secretly cut off Lucy's head and remove her heart. Although he doesn't provide much explanation, this is obviously because he suspects she has been turned into a vampire and wants to prevent her from rising from the dead.
However, the next morning, he wakes Jack up and says he has changed his mind:
"You need not trouble about the knives; we shall not do it." "Why not?" I asked. For his solemnity of the night before had greatly impressed me. "Because," he said sternly, "it is too late - or too early. See!" Here he held up the little golden crucifix. "This was stolen in the night."
I don't understand why Van Helsing has changed his mind, or how the stolen crucifix relates to his plan. From the events that transpire shortly afterwards, it seems his suspicion about Lucy was justified, and some trouble could have been saved by following through with the plan.
(please note that I am still reading the book and would prefer to avoid spoilers, if the reasoning is explained at a later point)