The whole atheist-Christian “can morality exist without God” debate is a mess, because there are actually three debates going on and being endlessly conflated with one another. So, following atheist philosopher Peter Singer’s lines of division, here is what’s really going on.
2. Do people need an authority figure (such as God) to force them to be good? This is where arguments like, “Are you telling me that if you stopped believing in God, you would be out raping and pillaging” fit.
3. Do people need an objective reference point (such as God) to understand what is good? This is, I think, the main Christian contention, that without something ultimately more real than nature, humans are just sophisticated animals who act for their own gain, and “morality” comes down to, “Let’s agree not to kill each other so that we can get some other stuff done.” If this is the case, deviating from the agreement not to kill each other might be tactically unwise, but isn’t wrong in an ultimate sense. Yet many of us feel intuitively and existentially that murder is not just inexpedient, but wrong all the way through.
Each of these subjects is a distinct line of argument. I think atheists actually have very good resources for answering each of these, but I very rarely see them deploy these resources. Rather, what I see looks more like a shell game, where anytime a question becomes too hard to answer, the atheist in question defers to an easy answer to one of the other two questions.
So this isn’t an argument, just an appeal to keep our arguments neat and orderly, as they should be.