If all demons are believers, then why are some humans atheists?

In the Bible, the demons are the angels who chose to follow Satan in rebellion against God. Therefore, having seen Him "face to face", it is foolish for the demons not to believe the truth about God. The demons chose rebellion not because they didn't believe in God, but because they wanted to usurp the supreme power and sovereignty that He holds.

There are only two ways to hold power: Take it by force or by successfully proving one's right to sovereignty. The former was/is impossible, so Satan tried the latter by first causing into question God's right to rule and then suppressing anything truthful about God's identity. "The father of the lie" has perpetuated lies about HIM since the Garden of Eden. Thus, many atheists exist simply because they haven't been properly exposed to unchallenged truth. Others exist because they choose not to believe.

The question appears to be premised on two main conditions.

1. Vague concepts of spiritual terms or identities. Satanic, fallen angel, demon, demonic and Hell, are commonly thrown around. This is odd to have so little precision, when verbalizing things so central to the plot of Christianity. Some stories need a good villain, or you don't have a good storyline.

*The first step is define demon. There are multiple paths this could take. Each will significantly alter the storyline.

2. The question shows itself to be influenced by it's place in time. In the West, specifically Britain and the Colonies, the 1700's produced revival movements. This era introduced a personal salvation, with a personal savior. Salvation became personally holding to concepts or opinions. Belief in God or unbelief, can simply be a change in opinion, yet one that alters eternal destiny and relation to God.

*The question imposes this same type of concept onto entities vastly different from a modern Western individual, such as an Evangelical.

*The question does not factor in the near censorship, but not total censorship, of specific entities by Scripture.

A partially censored statement occurs in Mark 1:23-25. A complete statement follows in Mark 5. While short, the voice describing itself as legion, goes beyond just providing personal details. This event is our only example of a petition, or prayer, brought by an unclean spirit. Belief or unbelief is irrelevant to their status. The spokesperson is mentioning an un-adjudicated status held by the spirits. This has been overlooked by Christian Orthodoxy.

Because of the statement being a petition or prayer, it was allowed to be made in full. Both Mark 1, and 5 identify the same position, but Mark 1 is not a petition. Presumably, the voice had no intention of bringing one, since it was silenced, mid-statement.

You've misinterpreted the verse you cite and become hopelessly lost. The topic is faith versus works, which is itself already based on misinterpretations of the scriptures and gospels, and the reference to "demons" is simply to a being (however hypothetical) that is universally and undeniably understood to be evil in order to contrast that with people of the time who claimed they were justified by faith (salvation by thought alone justifies any action) but their actions suggested otherwise. To take this form of argumentation as an ontological statement, much less an accurate one, is not only erroneous but harmful.

The Lord our God is One. There is no Rock; I know not any.

If all demons are believers, then why are some humans atheists?

I would like to build on the 3 fine answers below:

• Isaac Middlemiss's answer highlights the different epistemic ability of demons vs. humans, hence the impossibility of atheist demons
• Dan Fefferman's answer highlights how angelic demons (as opposed to human ghosts) use their knowledge for deceiving and detracting humans from redemptive faith
• Q34t's answer highlights the difference between epistemic certainty in the existence of God vs. God-provided saving faith that God (seemingly) give only for humans (because we don't know whether there is a saving plan for angelic demons).

We have to distinguish the ambiguous meaning of "believe" in the English language:

1. believe as knowing for certain (epistemic) vs.
2. believe as entrusting ourselves to the love of another and also reciprocate with love (like in marriage).

In the Biblical Hebrew language, the word "know" (yada) also connotates a personal and intimate relationship (as in Gen 4:1 and Gen 18:19). With this background, James 2:19 asserts that demons have this epistemic certainty and therefore shudder at the more powerful and immense creator God to whom they owe their existence, but obviously they don't love and trust God as someone who is good for them, evidenced by their attempts to oppose God's loving work for humans as the Bible demonstrates. Their deeds are not love and mercy, which is the larger context of James 2:19 when James rebuked some rich "believers" who dishonor the poor or unmoved by their basic needs (cf vv. 1-13 and vv. 14-16), thus equating them with demons who have useless epistemic certainty (since they have no love) !

But some atheists / agnostics who are still living and breathing on earth are operating under incomplete and false notions of God and the gospel (in contrast with hardened atheists who make a firm culpable decision to reject the gospel). Christian Inclusivists believe that:

1. after death they will be presented with the same epistemic knowledge that demons already have when they were created,
2. because of our temporal nature, God is revealing Himself piecemeal throughout an atheist's lifetime and is waiting until after death to make His particular judgment on him/her,
3. if because of life circumstances the atheist never had the opportunity to accurately comprehend God's love for them to make a rational decision to accept God's offer of salvation (let's say abusive upbringing, hypocritical/judgmental Christians around him/her, intellectual prejudices they were unable to overcome, etc.) God will take those circumstances into account, and
4. regardless of their imperfect epistemic knowledge while on earth, no one can remain neutral regarding the God they can only know partially. They either say to God "Thy will be done" (expressing honor for God) or "Thy will be done (with)" (expressing hate for God or what God stands for), evidenced by the quality of love for the other humans within the limit of their incomplete knowledge of God (i.e. do they forgive and wish their enemies what's good for them, not just their friends?). Catholicism teaches that this fundamental orientation (which flickers like a needle around the 0 point across one's lifetime) is frozen at the time of death (no more information mediated through their body). Their response to the full epistemic knowledge will be congruent with the needle's position: needle at negative condemns them, and needle at positive takes them to purgatory.

Conclusion

Yes, demons have a much easier time believing that God exists (because of their nature as pure spiritual beings). Yes, on judgment day all humans will too be granted the ability to believe with ease that God exists. Yes, there is a fundamental epistemological difference between demons and humans-on-earth before death.

Of course God knows the difference in humans vs. demons' epistemic ability and therefore judge demons immediately but judge humans over a temporal lifetime (as humans can change their mind depending on their changing rational judgement as more information becomes available).

BUT the criteria for judgment vis a vis decision to love God is the same. For humans, we are given a lifetime to ponder the redemptive offer from God through Christ Jesus, to which we respond either with love or hate.

It's pretty simple, actually.

Then Jesus told him, “Because you have seen me, you have believed; blessed are those who have not seen and yet have believed.”
John 20:29

Doubting Thomas just finished exclaiming, "I can't believe until I see the wounds in His hands and put my fingers in them." Then Jesus appeared and showed him. The rest of humanity won't get this offer.

Instead, we're offered the greatest blessing this side of eternity, which is to believe by faith. No other creature in all of creation is offered this great blessing, demons and angels included. Just another reason to pity the demons. For they see and believe and somehow still reject the Almighty.

In the passage quoted James is contrasting belief in particular facts with having an appropriate relationship with God—in English we might contrast “belief in” and “belief that”. (There is an old joke that plays on this distinction “Do you believe in the devil?” “No, I believe in God.” The question is “Do you believe in the existence of the Devil?” but the answer purposefully misunderstands it as, “Do you put your trust in the devil?”)

Inasmuch as we have no reason to assume that supernatural beings are engaged in gross deception, we are justified in concluding that all supernatural beings have access to the fact of God's existence, and accept it as a fact.

Therefore (James says) we shouldn't be so smug about having the theological facts down correctly, if we're otherwise living our lives in rebellion to God.

A further observation is that in Western culture, correct belief is generally held to precede correct behavior. In the world of the Bible that is not necessarily the case. For instance you can read in Romans 1 how Paul says that the nations were made foolish because they had rejected knowledge of God. So there at least (also all over in the book of Proverbs), the moral decision to accept or reject God is logically prior to believing specific facts. (This is admittedly not easy to translate into modern Western modes of thinking.)

Seeing is Believing

It's simple. Demons, like the God described in the book Christians read, are just as elusive in nature. You have to already believe to "see" evidence of them. It's like the final book in the Narnian chronicles. If you aren't already predisposed to believe in Aslan, you'll never see him.

Atheists and all other rationalists are going to go by evidence of their senses. Without proof of that type, there's nothing for them to believe in.

Interesting question for sure, and I would like to add some perspectives that I feel other answers do not cover sufficiently.

Are there fundamental epistemological differences between demons/angels and humans?

I think this is at the core of your question. A related concern which I feel is helpful to the answer is the difference between belief in the existence of God and saving faith.

1. Demons are supernatural beings

This has been covered fairly well by other answers. But to add a few Scripture references (probably not exhaustive): it seems that whenever Jesus encounters demons (devils (greek daimon and derivatives), unclean/evil spirits), or someone else mentions His Name, they know Him and his position (which is hidden even to the people around Him).

Matt 8:29/Mark 5:7-8/Luke 8:28-29, Mark 1:34, Luke 4:41, Acts 19:15

The assumption is that God (and Jesus at least before His incarnation), the devil and other demons would have knowledge about each other due to their supernatural nature. (Also see e.g. Luke 10:17-18, John 8:56-58 for some hints in this direction.)

2. Salvation by belief in Jesus is available only to humans

This seems to be the central theme of the Bible: the fall and restoration of Adam's descendants. The book of Hebrews paints (with quotes from the Old Testament) Jesus (the Son of God) as superior to angels (serving spirits) (Hebr 1) and then calls the saved humans His brothers (siblings) (Hebr 2).

Quote Hebrews 2:16:

So Jesus helps Abraham's descendants rather than helping angels. - God's Word

For of course it is not with angels that he is concerned; instead, it is with the seed of Abraham that he is concerned. - Unlocked Literal Bible

For surely his concern is not for angels, but he is concerned for Abraham's descendants. - New English Translation

This of course assumes that demons are of similar nature than the angels spoken of here. What applies to (good) angels applies even more to (fallen angel) demons.

Furthermore, 2 Peter 2:4 seems to indicate that those fallen angels have already been "found guilty" and only wait for sentencing. Again, assuming these are the same as the demons mentioned elsewhere (which do, in some of the previous references, speak of a certain time they will start undergoing punishment), but leaves open the question whether "hell" and "chains of darkness" still allow them to roam the earth as we perceive it.

I believe from the foregoing that there is no plan for salvation for the fallen angels/demons as they have been found guilty already.

3. Uselessness of demons' faith

Evangelical theology has the central tenet that we are "saved by faith" (as opposed to following the law). Rom 4 expands on this on the basis of Abraham's faith. But, as the section from which you also take your verse, Jam 2:14-26 shows, simply believing in (the existence of) God is a dead faith that can not save (verse 14). People need to act on that which they say they believe. Demons/evil spirits' acts are evil/destructive by definition when they afflict people, so I would argue that shows the nature of their faith.

Some other verses that come to mind regarding acts following faith, are in Acts 2:37-38 when the fresh believers ask what they should do, and Peter tells them to repent and be baptized (the same pattern repeats multiple times in various forms throughout Acts). Also Eph 2:8-10 contrasts the two kinds of works nicely.

As to atheists, 2 Thess 3:2 tells me that not all people have faith. My experiences with atheists have left me with the impression of the recurring theme that they won't believe without proof of God's existence. Do you see the irony? If something is proven as fact, there is no need/place for faith any more John 20:27-29, Rom 8:24, Heb 11:1. And I agree with the evangelical stance that salvation comes through faith, because this is explained in so many places in Scripture. I don't know why God chose to do it that way, although I certainly can speculate, but that would seem to make it unlikely that people would gain either saving faith or have time for good acts at the end-time revelation of God.

The text in question states that "even the demons believe" not that "all demons believe." One can certainly picture atheistic spirits wandering in the nether gloom, perhaps unware they are not still alive.

But perhaps James does not have formerly human spirits in mind. In that case the demons in question must be fallen angles, who surely retain memories of God's reality. However, even if all demons indeed believe, the cleverest of them might want to influence the rest of us to deny God's existence. This is one of C.S. Lewis' basic points in the Screwtape Letters. The demons themselves believe, but their goal is an atheistic, totalitarian world:

In a word, the Future is, of all things, the thing least like eternity... Hence the encouragement we have given to all those schemes of thought such as Creative Evolution, Scientific Humanism, or Communism, which fix men’s affections on the Future, on the very core of temporality.

This makes sense. The Devil knows God exists, but he does his damnedest to try to influence the rest of us to deny this reality.