Non-believingly is an adverb that describes something done without believing or without faith. Synonyms for the word include doubtfully, skeptically, uncertainly, hesitantly, and incredulously. If you are speaking about how someone speaks or reacts to something, you may use words such as sarcastically, disparagingly, or dismissively. If you are using the word to describe a lack of trust in something or someone, possible synonyms include distrustfully, untrustingly, and suspiciously. Keep in mind that the choice of synonym may depend on the context and intended meaning of the sentence, so it is always important to consider that as well.