The pertinent question may not be whether or not one can have a knowledge of right and wrong without religion; the question is, without the acknowledgement of a power higher than yourself, what is there to compel you to do right and abstain from doing wrong?
If you answer, “well, the state is a higher power, and one fears being punished”, then you have already admitted the flaw in the humanistic argument because, taken to its logical conclusion, the innate human comprehension of right and wrong would preclude the necessity of any penal system, since you could use the exact same argument they use against religion as an argument against the state: “All human beings possess an intrinsic understanding of fundamental morality, therefore laws are superfluous except as tools to control and oppress people”.
But no, the fear of God compels us to do right and abstain from doing wrong, even in the absence of a state, even in the absence of laws; and the truth is, that fear is just as innate as the core understanding of morality.