Is Satan the main force behind atheism?
Yes, and no. Ultimately, the "main force" is human rebelliousness... but Satan, in his desire to interfere with God's plan, surely does everything possible to help that along.
Scripture teaches that, ultimately, humans are rebellious by nature, wanting to be our own gods, and resisting the proper authority of the True God. By nature, we love what is Evil and hate what is Good (John 3:19). One who properly acknowledges God must acknowledge this, and must acknowledge God's proper Authority over their lives. This is contrary to human nature, and the way one avoids these objectionable actions is by denying that God exists. Scripture teaches that this is a willful process (Romans 1:18).
Moreover, God Himself knows who is unreachable, and, so that their guilt might be plain, will Himself act to harden the hearts of unbelievers (2 Thessalonians 2:11-12) and to weaken their consciences (Romans 1:24-32)... and these, in turn, also encourage others. It's important to understand, here, that we are speaking of people who had already rejected God, but surely God knows those who will accept Him and those who won't. Therefore, it should not be surprising that He arranges for those who were already condemned to be condemned utterly. When they stand before their Maker and all pretensions are stripped bare, those who rejected God will not be able to claim that their faults are minor.
Scripture also teaches that those without God are twisted in their reasoning (Ephesians 4:17-18), and that their "wisdom" is in fact folly (Romans 1:21-22).
[Satan] (or his demonic agents) must intervene in the physical world somehow to convince a person to embrace atheism.
I don't see how this is necessary. Mankind Fell because Eve listened to Satan's words, when he asked "Did God really say...?", causing her to doubt, and to believe that she could usurp God's prerogative of judging between Good and Evil. This same question, and the same technique of sowing doubt, continues to be the adversary's most potent weapon. Atheists doubt that God Created them, or that God even exists. Satan doesn't need to intervene physically to sow such doubts; indeed, that would be counterproductive, as causing disbelief in the spiritual seems to be one of his most effective weapons!
[Satan] must execute this influence in a masterfully subtle manner, ensuring the individual remains completely unaware of the spiritual manipulation prompting their atheistic beliefs.
Perhaps, but as others have noted, it's hard to notice what one rejects. In fact, observer bias is a well known effect. Add to this what Scripture says about the faulty reasoning ability of unbelievers, and it's not hard to see how atheists can be extremely resilient to challenges to their worldview.
How do Satan and his demons manipulate individuals into embracing atheism (or agnosticism) while remaining completely undetected? What are the mechanisms behind this imperceptible and subconscious manipulation?
(Honestly, this should probably be a separate Question, but...)
Certainly, Scripture says that the [spiritual] forces opposed by God have the ability to directly speak to humans. What's less clear is how obvious this action needs to be; the stereotypical notion of a disembodied voice seems plausible, but what about an overheard snippet of conversation, or a chance encounter with a seemingly normal person (actually a demon in disguise) that says something influential? Remember, we've established previously that it's in the adversary's (current) best interests to act with covertly rather than overtly, and here we have at least a few possibilities for influence that are both inconspicuous and don't require the ability to reach directly into one's head.
However, many believe that such "direct" influence is possible. This article, for instance, goes into some detail including the citation of several passages of Scripture. It isn't hard to imagine such manipulation being carried out carefully and with enough subtlety so as to not be noticed, especially when the manipulation aligns with the recipient's own desire to deny God.
As already suggested, however, one of the adversary's most potent weapons is peer pressure. How simple to stir up anger in a person with the intent of causing them to lash out at the perceived "foolishness" of another who is doubting the "orthodoxy" of atheism! Does Satan even need to get involved? Just look around! The pressure to conform to the material worldview is everywhere, and, to one unaware of the spiritual battle happening "behind the scenes", the source seems to be merely other humans.
Is there a means within Christianity to unveil and expose this influence, bringing it to the individual's conscious awareness?
I would say the best defense is to immerse one's self in Scripture. The first and most important step is to understand that a) there is a spiritual realm, and b) there is a battle being waged there over men's souls. (I use "men" in the historic, inclusive sense, of course.) To some extent, I am not qualified to answer, as I feel confident I am not myself aware of all the influence to which I am subject. On the other hand, armed with the intellectual knowledge that the spiritual battle exists, I find myself suspecting spiritual influence whenever I see humans acting against God's Will. Thus, I think the best answer I can give is to try to be receptive to what God says... and that is the true challenge of Christianity. It isn't a problem with a simple solution. However, the best tool we are given is God's Word, in the form of Scripture, and second to that, the Church. Try, too, to be mindful that Scripture is the only sure source of Truth; all else is tainted by man's ideas. Try, when reading the Word, to set aside your preconceptions about what is true and what is false.
So... read the Scripture, pray, repent, read the Scripture, accept the Sacraments, surround yourself with like-minded believers, and read the Scripture. In other words, be the best Christian you can be.
To my point, the mechanism through which the temptation is delivered must be the same regardless of what the prospective sin is, right?
- I don't know. At least in the case of over-eating ice cream vs. writing atheistic philosophy papers (e.g. see this example), I don't see how it is obviously the case that the exact same procedure is followed. If you know the mechanism(s), and are knowledgeable enough in the matter to explain it, please by all means post an answer.