incredulous (adj.)

• agnostic
• cautious
• disbelieving
• distrustful
• doubtful
• doubting
• dubious
• faithless
• guarded
• heretical
• hesitant
• irreligious
• leery
• mistrustful
• questioning
• quizzical
• repudiative
• sceptical
• skeptical
• suspecting
• suspicious
• unbelieving
• uncertain
• unconverted
• unconvinced
• unpersuadable
• unsatisfied
• wary