The traditional Christian claim is that God has imbued humans with a moral sense. If one were to take this claim as true then clearly each and every human - no matter their particular beliefs - would have the capacity for some sense of morality. Christian thought also claims that the very source of what we define as good - our sense of good - is what we call God (i.e. that which is ultimate and the source of all things). Otherwise - so the reasoning goes - how do we explain having the concept of good or that some things/acts/ideas are what we call good? If there is no source of good then claiming morality can seem arbitary.
It was this kind of reasoning that did lead some certain types of atheists (historically speaking) to claim that if "God is dead, then everything is permissible". Some of these atheists attempted to construct a new type of morality. One where individual morality was less important than the direction of society as a whole. The culmination of this type of atheistic thought (in the sense of a negative way) was found in Soviet Communism (particularly under Stalin) and such people as Mao Tse Tung and Pol Pot. These particular atheists and the systems they represented saw the individual as unimportant. It was the dialectic or the societal construction that mattered. Individuals that threatened or challenged this system were to be cast aside. Often, large swathes of people who stood in the way of progress were to be cast aside as well. As many of you may be aware this thinking led to the deaths of many millions of people in the 20th century. It represents one particular strand of atheistic thought and had some terrible consequences.
Of course I don't want to suggest that this means that all atheism (or even most) leads to this type of behaviour or thinking. The Thomas Paine, Spinoza inspired strand of atheist morality is an enviable, cultured and entirely agreeable system of thought with the rights and responsibilities of individuals bound right up into the system. A lot of the very good secular systems of government we have today have been influenced by the writings of such prominent atheists, agnostics, deists and other free thinkers. It is true to say that atheistic or agnostic leaning ethicists today do believe in a form of relative morality. But this a relative morality formed by reason, consideration and cause and affect. It is formed by looking at the effects of certain actions and the type of result that this causes within societies or towards individuals.
There is more one could say of course. These are complicated issues. Hope these thoughts are useful at the least.
