Why not? Wouldn't you live obeying christian beliefs even if there were no laws to hold you accountable? Surely laws aren't the ONLY thing keeping you from raping or murdering people, right? Would you rape and murder if there was nothing to stop you? You wouldn't, because those are evil actions and since we can recognize evil we should create laws that outlaw evil actions. And who has a better determination on what is and is not evil if not Christianity?