unbelieving (adj.)

• atheist
• atheistic
• disbelieving
• distrustful
• doubting
• faithless
• gentile
• heathen
• heretical
• incredulous
• irreligious
• mistrustful
• pagan
• questioning
• quizzical
• repudiative
• sceptical
• skeptical
• suspicious
• unchristian
• unconverted
• unconvinced
• unpersuaded