Nihilism (from the Latin nihil, nothing) is the philosophical position that values do not exist but rather are falsely invented.
Most commonly, nihilism is presented in the form of existential nihilism which argues that life[2] is without meaning, purpose or intrinsic value.
Christianity states that all is created by God. If we are God's creations then what value do we have?
Christianity also asserts that life is a filtration process in which humans are separated into to categories of "good" and "bad" in which the "good" go to heaven and the "bad" go to hell. If our ultimate purpose is to go to "heaven" or "hell" then what purpose do we have at all? If a chicken crosses the road and it's only reason for doing so is "to get to the other side" did the crossing of the road have any meaning? And if the chicken's only purpose in life is to cross the road does the chicken's life have any value?
Christianity consistently de-values life by describing it as the conveyor belt to the next one. They are dissatisfied with this life so they create a perfect one in their imaginations that they promise to each other granted that they don't "sin" i.e. make one and other unhappy in this life.
In this sense, it is the philosophical equivalent to the Russian political movement: the leap beyond scepticism — the desire to destroy meaning, knowledge, and value. To Nietzsche, it was irrational because the human soul thrives on value. Nihilism, then, was in a sense like suicide and mass murder all at once. He considered faith in the categories of reason, seeking either to overcome or ignore nature, to be the cause of such nihilism. "We have measured the value of the world according to categories that refer to a purely fictitious world".[20] He saw this philosophy as present in Christianity (which he described as 'slave morality'), Buddhism, morality, asceticism and any excessively skeptical philosophy.
all quotations taken from: http://en.wikipedia.org/wiki/Nihilism