How long until you start questioning if it will work and invite doubt?

Softly, almost purring, Barlow said, “Then will you throw away your cross and face me on even terms—black against white? Your faith against my own?” “Yes,” Callahan said, but a trifle less firmly. “Then do it!” Those full lips became pursed, anticipatory. The high forehead gleamed in the weird fairy light that filled the room. “And trust you to let him go? I would be wiser to put a rattlesnake in my shirt and trust it not to bite me.” […] Barlow seemed to grow taller. His hair, swept back from his brow in the European manner, seemed to float around his skull. He was wearing a dark suit and a wine-colored tie, impeccably knotted, and to Callahan he seemed part and parcel of the darkness that surrounded him. His eyes glared out of their sockets like sly and sullen embers. “Then fulfill your part of the bargain, shaman.” “I’m a priest!” Callahan flung at him. Barlow made a small, mocking bow. “Priest,” he said, and the word sounded like a dead haddock in his mouth. Callahan stood indecisive. Why throw it down? Drive him off, settle for a draw tonight, and tomorrow— But a deeper part of his mind warned. To deny the vampire’s challenge was to risk possibilities far graver than any he had considered. If he dared not throw the cross aside, it would be as much as admitting…admitting… what? If only things weren’t going so fast, if one only had time to think, to reason it out— The cross’s glow was dying. He looked at it, eyes widening. Fear leaped into his belly like a confusion of hot wires. His head jerked up and he stared at Barlow. He was walking toward him across the kitchen and his smile was wide, almost voluptuous. “Stay back,” Callahan said hoarsely, retreating a step. “I command it, in the name of God.” Barlow laughed at him. The glow in the cross was only a thin and guttering light in a cruciform shape. The shadows had crept across the vampire’s face again, masking his features in strangely barbaric lines and triangles under the sharp cheekbones. Callahan took another step backward, and his buttocks bumped the kitchen table, which was set against the wall. “Nowhere left to go,” Barlow murmured sadly. His dark eyes bubbled with infernal mirth. “Sad to see a man’s faith fail. Ah, well…”

-Stephen King, Salem’s Lot