Richard Dawkins arguably gave religion a far better explanation and dissection in his 1976 book the Selfish Gene than he ever did in the God Delusion. The Selfish Gene may lack the polemical nature and the specifics of his recent religious commentary, but it nails on the head the key behind religious belief and apostacy. This of course is the book where he coined the term "meme" and memetic evolution, and he employs religion as one of his most common examples of these both.
Naturally it's not only true that religion functions as a bit if cultural heritage, but as Dawkins explains, memes have a tendency to link themselves together, sometimes prolinging their lifespans by association.
Building off that, if we say that the concept of a creator god is one meme, it should be obvious that religion is the framework that links that meme with others: a meme that expounds on what behaviors are permissible, a meme of a specific creation mythology, a meme for eschatology, a meme for proper family behavior and thousands of others. Thus in a way, religion is not so much a set belief that can easily be switched off, but abandoning the concept of god can easily undermine foundational moral and social beliefs as well. To change your religion is to totally alter your memeplex, with a desperate need to replace not only factual beliefs, but basic social understanding.
In a more personal way, if someone becomes an apostate, they would lose a lot of the religion-specific justification for their family life, their moral persuasions and likely even their politics. Religion is so powerful over people's lives because it connects with so much of the foundations of their everyday interactions, perhaps not practically, but ideologically, and it aids in putting human action unto context given the wide worldview attached to religion.
Although it sounds practically Kuhnian, discussions of religions are not so much mediated by the facts, but by the immense ideological threat non-belief poses to someone whose life relies on religion to explain and make meaningful the rest of life. Both the religious and the non-religious know what side is "right" before they approach any issue, and this is mostly due to religion's memetic ubiquity.
This should also give us insight into what conditions make religious people more likely to leave their faith. Convincing counter-apologetic arguments may be a prerequisite, but seem obviously insufficient alone. Instead, people are more likely to lieave their faith if religion's other memetic allies have already been compromised. That is, if they lack a religious family or friends, if they don't participate in any services and if they don't invoke religion to justify moral or political actions, they'll probably be able to disbelieve in God with greater easy. Atheism becomes relatively costless. A person in these situations frankly wouldn't be too troubled by the idea of a godless universe.
Religious people are variously dispersed between having religious beliefs that interlock with every other belief or meme of theirs and having religion as a thenuous and localized meme that could be switched on or off on demand. A lot of the difference between fundamentalist and liberal Christians or between moderate and extremist Muslims can be accounted for by how much of the rest of their beliefs are interlocked or influenced by the meme-network of religion. Fundamentalists and extremists view religion as fundamental to understanding anything in the world, thus in their minds, it would be misguided, unintellectual and oblivious not to have religion guide them in social affairs. On the other hand, believers with a tiny and insular memetic network for religion are likely the ones to want to keep religion seperate from politics and everything else, something which is a form of denial or stupidity to truer believers.
Everybody's error has been to understand religion as one or a set of beliefs. It is incredibly rare or likely non-occurring for a person of fervorous religious disposition to hear one or two off-the-cuff arguments against God's existence and then on the spot relinquish their religion. It's an issue of intellectual economy and the ideological support of other associated memes. Perhaps an argument can cause some questioning about the existence or justice of God, but at the same time it would be unlikely for a believer to toss out the God meme and every meme associated with it, as to them, the testimony of their family, church life and moral convictions have not been rebutted and thus are supposed to constitute evidence confirming their ideology. Of course in a way, religion survives because the memes of religion have taken the memes of morality and social cohesion as hostages, and for the religious, losing the former means losing the latter.
The paragraphs below are a later addition of LS:Everybody's error has been to understand religion as one or a set of beliefs. It's incredibly rare, or just never the case that a person of fervorous religious conviction hears one or two off-the-cuff arguments against God's existence and then relinquish their religion on the spot. It's an issue of intellectual economy and the ideological support of associated memes. Perhaps an argument can cause some questioning about the existence or justice of God, but at the same time it would be unlikely for toss out the whole memeplex at the first sign of complexity. To religious people, God is evidenced by the whole memeplex: their relationship with their family, their church life, their moral convictions and everything else.
I will add that this is part of the reason I think that trying to convince "irrational" religious people is somewhere between useless and destructive. The idea that religion is a set of beliefs is a novel one, and several centuries ago (maybe before the Protestant Reformation) it was generally understood that religion was a system of social cohesion with some epistemological elements. Trying to get people to toss away the entire social cohesion system because some epistemological statements are wrong not only misses the point, but can cause unnecessary social upheaval if it actually succedes.