Consider this simple thought. What exactly is a believer? People today assume that belief means to acknowledge the existence of a Being, but even the most evil Beings in the creation know that God exists. Would you call an evil demon a believer? Of course not.
Contrary to modern opinion, to believe in something does not simply mean to believe it exists. Do you believe terrorism exists? Would you ever dare to say you "believe in terrorism"? Of course not. Only a terrorist would say that.
People make logically contradictory claim about themselves all the time. An evil person may believe that a deity exists but that does not mean they are a believer in that deity. Someone who has dedicated their life to evil by definition cannot logically believe in a deity whose values are opposite of their own. When people "believe in" something, it means they believe in what it stands for, not merely that they believe it exists. To worship does not mean to simply bow down - to worship implies devotion, dedication, and modeling ones ideals after the object/Being of worship. In a similar sense to how modern parents often accuse their children of "worshiping" celebrities or musicians, in instances where the child might start dressing, acting, or talking like the celebrity they idolize. An evil person cannot be said to worship a benevolent loving Being - that idea is a logical contradiction.
The distortion of this definition is an obvious symptom of the Kali Yuga - where people who consider themselves the most righteous, will often end up being the most corrupted and arrogant, ignorant to their own corruption. We can see this corruption everyday: where people often accuse others of being bad/evil because they do not believe in their particular version of some deity, even if the people might have identical moral values - it is the epitome of shallow vanity and false piety. Do you think the evil Beings whom Krishna slayed believed in his existence? Of course, they obvious knew he existed, but they were certainly not "devotees" in the sense of the word, otherwise they would be evil and he would not have slayed them.
This concept is not as foreign as it might seem. For example - even in the bible Jesus declares that he will reject many people who come to him as Lord due to the iniquities/evils they dedicated their life to, regardless of how much time they spent praising his name. Likewise, there are parables that Jesus taught where he indicates that the false believers he rejects will have their spots taken by good strangers who never knew him.
While this obviously isn't a Christian forum, this notion of what it means to believe is inherently obvious when closely looked at. Someone who is a morally good Atheist can certainly be considered a believer in a manner of speaking, with respect to the fact that they have dedicated their lives to values that a benevolent deity requested of people.
Basically in the most general sense, when it comes to labels like this, we are defined by the actions and desires of our hearts. Most notably - people today forget that words in Sanskrit are only ever valid when they accurately describe the properties that an object has. If a word in Sanskrit is describing some moral quality, and a Being does not possess that quality, then that word can not logically apply to them. However that word would automatically apply to everyone who does possess that property.
During time periods when supernatural phenomena is obvious, this distinction about the nature of belief would never be required, because all Beings would inherently understand that supernatural phenomena exists in their world. Yet today we are in a time when very few people have ever known anything genuinely supernatural. People today often barely believe in things they witness with their own eyes. The definition of "belief" has become dramatically weakened.