what are we supposed to say again when christians ask us where we get our morality?

[Mudhammam]

That doesn't make a lot of sense. Atheism, by it's definition, implies that any metaphysical conclusions we come up with must be rooted in comprehended human experience. That's not nothing. That's reality--an offering to people that amounts to the freedom to think and search out one's own truths without fear of always being monitored and judged by a fire-breathing omnipresent mind--which is apparently non-physical, non-spacial, non-temporal, and every other negation of necessary properties that generally qualify an item said to "exist" as a "something." In other words, God as an unidentifiable, incomprehensible abstraction--a thought independent of a thinker--is literally the closest to "nothing" that the human mind can (or rather cannot) conceive.