Thanks so much for this interesting question about atheism...
The world abounds with conspiracy theories, but I have never heard of a serious one that involves atheism. Of course, if there were a serious effort afoot, maybe it would not advertise itself on the Internet.
To my understanding, atheism is a case of doubt as to the truth of the existence of God—not only doubt, but a conviction that there is no unseen Deity at all. Agnosticism allows room for doubt, but an avowed atheist is sure that there is no God.
I am sure that even persons who are honest of heart can be atheists. I have often thought, though, that a true atheist is one of the most spirit-minded of persons, as they devote their lives to dis-proving that which is not provable, except in the realm of faith and spiritual experience. Likewise, God's existence is neither able to be dis-proved. If there is such a conspiracy, I think that it will have little chance of ultimate success, as the overwhelming majority of people on Earth DO believe, and have always believed, in some form of Deity Presence.
The seeds of atheism were sown on this planet in the long-ago days of the Lucifer rebellion. Lucifer himself taught evolving man that there was no God, and this was one of the cornerstones of his infamous Manifesto. Unfortunately, we still see the repercussions of his insanity even today in the form of atheism as a "philosophy." It takes quite a long time for the effects of rebellion to be eradicated.
Because we do not have the spiritual helpers that a normal planet enjoys, such as Adam and Eve and a resident Planetary Prince, we humans must rely on faith and desire to find God. For some, this is a leap that they are not prepared to take, and so they prefer instead to rely wholly upon their material senses, which are hardly adequate to discern spiritual presence. Little wonder that atheism can flourish in a world that glorifies the scientific method divorced from spiritual insight.
In our 21st Century world, atheism seems to be enjoying a resurgence, but is it a serious threat? Is there an organized and concerted effort to undermine belief in God? I have not ever heard of that, but that is not to say that some atheists may desire such an effort, as they can be quite strident in their unbelief. There is also a "church" whose members worship Satan, but does that constitute a "conspiracy theory?" I think these are just confused folks who have yet to learn the reality—or the possibility—of God through their own experience.
Thanks again for your question; I hope that my reply has been helpful.