Appeals to popularity suggest that an idea must be true simply because it is widely held. This is a fallacy because popular opinion can be, and quite often is, mistaken. Hindsight makes this clear: there were times when the majority of the population believed that the Earth is the still centre of the universe, and that diseases are caused by evil spirits; neither of these ideas was true, despite its popularity.
Example
(1) Most people believe in a god or ‘higher power’.
Therefore:
(2) God, or at least a higher power, must exist.
This argument is an appeal to popularity because it suggests that God must exist based solely on the popularity of belief in God. An atheist could, however, accept the premise of this argument (the claim that belief in God is widespread) but reject its conclusion without inconsistency.