I think that having a belief in something outside of humanity as an underpinning helps to keep morality grounded and consistent (not that religious people will consistently act more morally, but that the actions themselves are likely to be viewed the same from year to year).
I think humans have an uncanny ability to logic their way into the acceptance of anything they truly want and therefore secular ethics and morality will often revolve around how many people you can convince to agree. There are no fundamental truisms, there is only what we create.
"Murder is bad."
"Well of course it is."
"Why."
"We have no right to take another's life."
"Why?"
"Because it isn't right."
"What about war? As punishment for heinous crimes? In order to protect your family? What if it is convenient? They tax your economic well being? They are handicapped?"
"We shouldn't steal."
"What if they are poor, are historically disadvantaged, and/or insurance will cover it anyway? What if we empower someone else to do it and call it taxes or redistribution?"
I think without God underpinning morality, it is a relativistic concept and if it is relative, then there is no such thing as true morality.