Christianity as inter-subjective system
The Raving Atheist asks why Christians and Hindus have legitimacy while belief in Poseidon would not have said legitimacy.
This is in fact not a problematic question at all. Religions are inter-subjective systems, therefore a specific religion gets its meaning and credibility from inter-subjective agreement. When such an agreement exists, this is a state that True Believers uphold as "true" (if they are part of the agreement) or "credible" (if they are part of a different, opposing agreement). Hindus and Christians both have strong established inter-subjective groups, while the Greek pantheon no longer does. Therefore Hinduism and Christianity are credible from the Believer perspective (and true within the group's adherents), while belief in Poseidon is not. If belief in Poseidon had its own inter-subjective system, then it would be considered credible.
In essence, the most ridiculous, absurd, offensive beliefs can be made credible by virtue of having a strong inter-subjective group. This is also why atheism is not inherently credible, as individual atheists are irrelevant as long as there is no systematized inter-subjective agreement between them. Atheism therefore only gains recognition insofar as it represents an attack on the inter-subjective game (more on this below).
Cults progressively become religions as they gain this credibility. In this passage, a system also mollifies its cult attributes, because they are no longer needed to attain credibility and become more of a hinderance. This, however, is only true when these systems evolve within a relatively free society. In theocracies, there is much less reason to relax cult attributes, as now the resources of an entire society can be used to enforce them. Christian sects, on the other hand, do not generally need to start as cults, because they already have the inherent credibility of Christian agreement.
Christianity specifically has all the marks of an inter-subjective system :
* Truth is determined by agreement, not by objective justification.
We observe this in all the Christian sects, and in their debates against each other. The standards used are taken from theology, which is itself another inter-subjective game. Ultimately, it reduces to a clash of inter-subjective readings of the same texts, where the credibility of a "reading" is given by strong agreement. For example, some sects like the Jehovah's Witnesses or the Mormons are considered "outsiders" because their "readings" stand out from the general agreement between most Christian sects.
* Meaning, purpose and morality are inter-subjective, more specifically utilitarian in nature relative to the system itself.
I have many articles coming in December on this topic. My thesis is basically that, as Christianity is a meme complex, its morality serves the perpetuation of Christianity, its institutions and its inter-subjective group. Therefore Christian morality is fundamentally utilitarian - it exists solely for the benefit of the greater system.
* Conceptualization shifts, sometimes slightly, sometimes dramatically, to fulfill the needs of the system's worldview(s).
In cult studies this is called "loading of the language". However, it does not need to take the dramatic turn it does in cults, where entire swarths of vocabulary are redefined. It can be relatively subtle, but the conceptual change is there if you look for it.
* The most important role of the agent is to align himself with other agents within the system.
Corollary : agents are accepted as long as they are aligned with the other agents, regardless of whatever else they did.
There is a certain pressure to conform within given sects, although break-offs do occur. Once again, this is to be expected, since there is no objective standard on which to resolve disagreements, and the domain of religion is faith-based and therefore inherently divergent (in fact, this is an epistemic argument against it). Theoretically, we should observe these break-offs in contexts where there is too much tension between the local environment and the sect's specific belief system.
* Major changes take place under the form of painful and long deconversions.
In a rational system, such as science, reversals of previously established models are long and arduous, but they are not of a personal nature. Rather, new generations come to the fore and, starting afresh, realize the truth of a new model and elevate it. In an inter-subjective system, however, adherence to a specific set of beliefs is a prerequisite to entry. From there, belief is reinforced and becomes habit. Because of this enforced agreement, and because there are no objective facts for the individual to rely on, major changes in one's religion are usually painful, uncertain, personal affairs.
* From the previous two points : global changes take the form of discrete breaks, instead of continuous adaptation.
* The religion game.
Religion in general is an inter-subjective game where, if one does not adopt "the right religion", it is still better to "play the game" - i.e. to have any religion. Collectivist games are played at many levels - the organizational level (competing religions), the doctrinal level (competing theological interpretations), and in some cases the personal level (changing religions to marry someone, for example).
Atheism is seen as the worst possible option, being a total abandonment of the game, and this is an attack on the legitimacy of the game of religion. As I said before, this is how atheism gets recognition - as "the enemy". Atheists, however, are a hard target because they are not organized. This is why science, secular humanism or human rights organizations usually make better targets for religious ire. Alternately, the threat can be deflected by stating that atheism is a religion. This makes it part of the game again, and restores legitimacy.
I hope I haven't forgotten any important point in this list. If I did, enter it in the comments and I will add it.