General Opposites:
* Indifferently
* Apathetically
* Coldly
* Dispassionately
* Lukewarmly
* Halfheartedly
Specific to Religious Context:
* Skeptically
* Unbelievingly
* Irreverently
Other Related Words:
* Doubtfully
* Hesitantly
* Reluctantly
The best antonym for "fervently" will depend on the context of the sentence.