The difficulty appears to lie in the assertion by believers that we can't know what right and wrong is without there being a god to tell us. The question is why not?
It seems to me that their only reasoning is because they believe it so.
Then thank goodness we don't have to rely on your conclusion that it is merely because we, "believe it so".
It's actually because of a fairly straight forward issue of where you find ultimate morality to begin with, in terms of it always being a relative or subjective matter.
By analogy there is a game called, "Scruples", but the problem is this; where did the makers of "Scruples" get their set of criteria?
Another example is this, there appears to be at least by human standards, various groups of people that are fairly decent by at least our own relative standard as humans, yet they may have a different moral compass.
So that is the problem, if God did not exist then basically if somebody murders you, to see this according to the implications of atheist materialism, it follows that strictly speaking all that happened when someone murdered you was that molecules collided.
You see this is the problem, if you're going to tell people they're ultimately just a material accident, then logically it follows that there really isn't any morality, in that if you die and you were murdered, there is no actual justice after death in a materialist scenario.
Under a strict evolutionary, materialist scenario, it quite literally would not ultimately matter to the universe if you were sliced, diced then thrown on the fire, or lived a life of paradise-like perfection.
Conclusion: These are the logical implications of a strictly materialist universe whether you like it or not. Your destiny under this philosophy, is that you have the same worth as a bowl of spaghetti, which is obviously absurdly false, because the value of a human sentient person, made in God's image, is well, obvious, and it is obvious that therefore the atheist belief is against the facts, because factually a human being has more value than merely it's material. (modo hoc fallacy).
But that is ultimately what you have to accept to be a consistent atheist. Ultimate, objective morality is not available, because like with the, "Scruples" example, who decides who is right?
But when you take THE OBVIOUS TRUTH, that only an all-knowing God can be righteous, and accept people are sinful human beings, you get a much more consistent, realistic picture of reality.
So if you strive in your mind to accept murder is just as lawful as making a cup of coffee, then you yourself in your mind are acknowledging that atheism is inconsistent with reality.