Not only is it wrong to claim that ‘objective’ ethics presupposes religious belief (just google ethics and atheism and you will have enough to keep you busy for a day), it may have the issue completely backwards. The real question is not whether an atheist can be commited ethics, but whether a person of religious faith can ever be committed to any ethic. In "Fear and Trembling" Kierkegaard assumes that ethics is the goal of achieving the universal good, which can brook no compromise. If this definition is true (see previous discussion here), Abraham’s desire to sacrifice Yitzchak at the akeidah must be deemed unethical and an act of attempted murder. Since this for Kierkegaard (and I think for any religious Jew) a reductio ad absurdum, he concludes that there exists some higher law which transcends even the universal good of ethics, namely religious faith. Obedience to ethical principle can be suspended to achieve the telos of obeying the higher law of G-d’s will – the teleological suspension of the ethical. Does that mean faith does not impose ethical standards? “This was not to suggest that from a religious point of view moral standards and principles could in general be abrogated or overruled. It did mean, on the other hand, that within that perspective they took on a radically different aspect, one where they possessed a relative rather than an absolute status and where it was the individual’s own relation to God that was paramount, assuming precedence over all other considerations” (article here, also see here).
Avi Shafran writes, “If our perception that some deeds are good and others are not is but a quirk of natural selection, none of us need feel any commitment to morality or ethics.” Quite the contrary – if ethics is a product of natural selection, we inherently and unequivocally will be committed to its principles the same way we obey other biological drives. However, faith always carries the trump card saying “Yes, your ethical argument makes sense, but G-d told me to do otherwise!” To rephrase Shafran, the atheist might say that if an ethical norm like not murdering is obeyed simply by quirk of its coinciding with G-d's will, none of us are truly committed to the ethic of not murdering, as we will not hesitate to abrogate it when our subjective assessment of G-d's will dictates otherwise.
I'm not out to debate this issue at length - Kierkegaard can be rebutted and many other views offered. All I know is that offering simplistic answers is not the solution to rebutting the claims of atheists or defending the richness of yahadut.