R. Aharon Lichtenstein (adapted by R. Reuven Ziegler), By His Light: Character and Values in the Service of God, pp. 119-121:
Regarding the philosophical argument, it is perhaps true that a strong case can be made for the notion that without God everything is lawful... But even if one were to concur with this philosophical argument, can we factually deny that there exist people who are totally removed from religion yet nonetheless act in accordance with high moral standards? Perhaps they are logically inconsistent; perhaps if they were deeper philosophers, they would be worse people. Yet they regard themselves, and we would regard them too, as moral individuals. We cannot be oblivious to the existence of this phenomenon. How, then, do we relate to it...?
[W]e surely should not dismiss nor denigrate moral idealism simply because it springs (in certain cases) from secular sources. Certainly, we believe deeply that a moral idealist would be at a much higher level were his morality rooted in yirat Shamayim, were it grounded in a perception of his relation to God and of the nature of a man as a respondent and obedient being. But that surely is not to say that we therefore ought to dismiss totally the possibility or the reality of secular morality. First, we should not do this because it is simply untrue--there are genuinely moral people within the secular community. Second, we ought not do this because, after all, the results are not what we should be seeking.