Why the religious engage and then withdraw in a discussion?

From Amazon's discussion forum:

People believe because they are afraid to not believe.

Fear of death is a powerful driving force common to all animals.
It is even a part of our physiology. When we are threatened we produce adrenalin and prepare to defend our lives. We do everything we can to avoid death. But human self-awareness means that we are able to consider the inevitability of our own death in the future. This is physically inescapable and must therefore create some sort of internal conflict or fear. One way of resolving this and `escaping' death is to form the belief of an afterlife. The physical body dies but we continue. Tension resolved. To deny this belief returns the tension and the fear and that is what makes it so hard for believers to give up their faith. They spend their whole lives shoring up their beliefs in order to keep the fear of death at bay.

The fear of atheists comes from the challenge to the certainty of their beliefs. If it is so clear that God, Jesus, the Bible, hell, whatever, is actually so obviously true, then how is it conceivably possible to not believe. It must create some doubt, however small, that must be expunged. If they dare to engage in discussions with atheists it is with the intention of invalidating any view contrary to their own. If the discussion begins to show up chinks in their armour they usually just disappear. Or they might side-step the issue at hand to avoid facing a possibly uncomfortable truth and, heaven forbid, any loss of faith. And it is not surprising that often the fear manifests itself as hatred towards those that do not hold the same cherished beliefs.