When it comes to describing someone who doesn't believe in a particular idea or concept, there are various synonyms that can be used instead of "lack of belief." One alternative is skepticism, which indicates a dubious or questioning attitude towards a belief or claim. Doubt and disbelief are two other synonyms that imply a lack of conviction or trust in something. Another term that can be used is atheism, which refers to the absence of belief in the existence of a deity or the supernatural. Finally, agnosticism is another term that can be used, indicating a lack of certain knowledge or conviction about a particular belief.