General Antonyms:
* Apathetic: Lacking interest or enthusiasm
* Indifferent: Not caring one way or the other
* Lukewarm: Showing little enthusiasm or interest
* Tepid: Moderately warm; lacking in enthusiasm or conviction
* Passive: Not actively involved or engaged
Antonyms Related to Emotion:
* Cold: Lacking warmth or passion
* Detached: Emotionally distant
* Unmoved: Not affected by emotion
* Stoic: Showing no emotion
Antonyms Related to Belief:
* Skeptical: Doubting or questioning
* Unconvinced: Not persuaded
* Hesitant: Unwilling to commit
* Doubtful: Uncertain or unsure
The best antonym for "fervent" will depend on the specific context in which the word is used.