When we consider the theory of religious belief being a basic need for ultimate protection and care similar to that provided by the womb and the parents in early infancy, it follows that a religion that provides this need most efficiently is more likely to spread and prevail than others that dont as adequately deal with this need.

The survival of the fittest theory is therefore also applicable to religions.

Though in principle each human being could live with his own self-supplied gods serving his needs, obviously people shared feelings, thoughts and experiences. They developed common concepts, feeling mutually strengthened by shared beliefs. These structured sets of shared religious beliefs can be deemed RELIGIONS. Over the ages religions were refined and improved, partly as internal changes, partly as sects sprouting from the main religion and partly as brand new religions. Those that proved more successful in supplying the basic needs prevailed. They spread rapidly and pushed aside most of the primitive religions.

The gods of the rocks and the trees of primitive cultures provided some of the protective parental shell needed, but they really stood no chance against the Judeo-Christian-Islamic god providing the ultimate Father (and Mother) figure acting almost exactly as the parents are perceived by the infant. Similarly, the ancestor-based religions of the Far East outweighed less defined primitive gods.

As I stated I my previous article, the human mind always attempts to find substitutes for real or perceived loss in order to regain its equilibrium.
The substitute found is often only partly effective and the equilibrium achieved is shaky. In spite of this the individual will cling to its solution and consider any suggestions to exchange it for a more successful substitute with great suspicion and fear, since even the temporary shake-up of the equilibrium during transition of substitutes constitutes an insecurity the individual can't cope with.
Therapists know though, that once this initial apprehension is overcome, the new solution, proposing better and more efficient means of achieving equilibrium, is gladly embraced by the patient.

Similar to this process, we are aware of the fierce antagonism between religions. A religion will usually explain this anger as a result of the insult afflicted to it's true god(s) by the defiance of that god by the infidels/heathens/etc.
Actually, the reason behind these emotions is that the mere suggestion that the "true" god of a persons religion is not really the true god, immediately creates fear of disruption of the equilibrium in each religious individual who believes in that particular god. In order to ward off any chance that such a disruption might occur, other religions are violently and often viciously attacked (whether by weapons or words).

And yet, conversion occurs, at times even in great numbers.
Just as happens with the patient in the care of a therapist trying to make him change his behavior, once the initial very strong apprehension of the new and different is overcome, and assuming the new religion offers more in satisfying basic needs, individuals and groups may certainly adopt and even embrace the new religion, finding a new and steadier equilibrium. The therapist and the converting religion alike, apply the same tactics: simultaneously stressing the assets of the new theories/religion to the persons well-being AND helping the person passing the transition period as smoothly and rapidly as possible, thereby minimizing the fear of loss of equilibrium.
With religion this is often achieved by instant cancellation of debts, i.e. all previous sins are forgiven and one starts with a new blank slate. The temptation of being able to wipe out all past deeds that otherwise occupy our conscience is so strong that, offered together with an attractive package of fatherly/motherly care, it might just convince the individual to make the jump.

For those of you who have read up on atheism and came across memetics, one short remark. There is really no contradiction between the previous paragraphs and the theory of memetics. Just that I perceive the memes as suggested in the memetic theories as too vague and mysterious.
I believe that the fast and vast propagation of certain religions simply lies in the effectiveness by which their concepts answer the explicit human needs as explained by the Embryonic and Parental Shell Theory.
(If you havent heard of memetics, no big loss, but if your interest is aroused you can read up on this in the FAQ files of the major atheist newsgroups on the Web).

The next (and last) chapter suggests different ways to debate with theists.

Different Ways to Cope with Theists