I am not responsible for bringing consciousness, happiness, or the sense of freedom into the universe, but now that they are here—and however they got here—I appraise them as very good things, and want them for myself and, by imaginative sympathy, for others. As it is a good thing for a bird to fly and follow its instincts for nest-building, so I appraise it a good thing for a human being to be free to think and act in the world in pursuit of her self-determined happiness, and to have imaginative sympathy for the happiness of others (whether God exists or not). And so I ground my morality in human flourishing: what makes for my own flourishing and what makes for other people’s flourishing?
In terms of moral guidance, I think that this is a better question to ask than, say, “What would Jesus do?”
And I agree with Sam Harris that the question of what makes for human flourishing has some answers that are, frankly, objectively better than others. And I also think that you can ground the question in things associated with critical thinking: dialogue, good reasons, logic, evidence, and investigation.
Now why I should value consciousness, happiness, freedom, and imaginative sympathy in the first place is, I think, an unanswerable question (either on theistic or atheistic terms), but I join in solidarity with others who share with me a love for these things. Their value is, at least to me, self-evident and axiomatic. If, however, you think that it is coherent to not value consciousness, happiness, freedom, and imaginative sympathy then you are free to team up with others who share your views, but I cannot be in solidarity with you.
My human moral aspirations are well represented in the secular European Enlightenment, in the ideals of Thomas Jefferson, and in the Universal Declaration of Human Rights:
If you don’t think that a mere preference for consciousness, happiness, and freedom—accompanied by a wish of these things for others via imaginative sympathy—are sufficient for sustaining a moral life, then what do you imagine religion adds, in terms of value, to the moral equation?