Because those opposed to Christianity truly believe that they will be able to craft a system of morality that is unconnected to any religious (particularly Christian) attributes. They believe that they will be able to tap into some innate core of "goodness", which exists separately from and religion.
I disagree. And this Front Page link explains why:
...the core Enlightenment values are inextricably tied to Christian values. This view has been put forward most forcefully on the Continent in recent years by Marcello Pera (former President of the Italian Senate, and an agnostic) and by Benedict XVI (not an agnostic). They have argued that the Enlightenment grew out of Christianity organically, as a tree grows from its roots. Cut off from its roots the tree dies.[my bolding]
In this view the rights of man are based on a belief in the importance of man. The belief that ordinary individuals have a value and dignity of their own apart from their membership in a tribe or a society has its origin in the Judeo-Christian declaration that man is made in the image of God. Thus, if you take away God, you take away the foundation of human importance. As Thomas Jefferson undoubtedly discovered while composing the Declaration of Independence, it’s a bit difficult to establish the case for human rights without reference to the Creator. Purely secular societies can only assume human dignity and human rights as a given. We tend to forget that these concepts are now a given because they were given to the world by Christians. Before Christianity, the idea that all human beings are endowed with intrinsic value was not considered “self-evident,” it was considered ludicrous. Espousing human equality was a good way to get yourself laughed out of polite pagan society. Human dignity may seem self-evident to us now, but that is because the Christian moral view became internalized over the centuries. Gladiatorial combats and slavery didn’t go out of fashion because societies evolved but because people began to see one another in the light of the Christian revelation.
This confirms something I've noticed. I have known several atheists. They were generous, humane, and thoroughly decent people. But, I am under no illusion that their goodness sprung from a void. They lived in Christian communities, and were influenced by Christian values.
Think of some recent societies that were actively ANTI-Christian:
- Nazi Germany
- Most Islamic countries
- Japan during WWII