Much of modern secular/socialist thinking about these matters goes back to Barak Spinoza. He was the forerunner of Friedrich Nietzsche, Karl Marx, Vladimir Lenin, Joseph Stalin, Pol Pot, Mao Zedong and Adolph Hitler.

“The consequences of Spinoza’s system also envisage a God that does not rule over the universe by providence, but a God which itself is the deterministic system of which everything in nature is a part. Thus, God is the natural world and has no personality….Spinoza also held that everything must necessarily happen the way that it does. Therefore, humans have no free will.”

“Spinoza held a relativist’s position, that nothing is intrinsically good or bad, except to the extent that it is subjectively perceived to be by the individual. Things are only good or evil in respect that humanity sees it desirable to apply these conceptions to matters….Given Spinoza’s insistence on a completely ordered world where “necessity” reigns, Good and Evil have no absolute meaning. Human catastrophes, social injustices, etc. are merely apparent.”

http://en.wikipedia.org/wiki/Baruch_Spinoza

“If God does not exist, then everything is permitted” Fyodor Dostoyevsky