If there is no God, why be moral?

I daresay it is revealing that the question takes this form. I make these two points: this unsettling stance firstly implies that he who asks is some kind of monster who is only moral due to having been enjoined not to act wickedly. Moral for no other reason than a concern for your eternal soul, you conduct good works while thinking privately that you care nothing for the actions your physical self is performing.

You don’t mean it when you compliment a person or provide them a gift; you don’t do favors for others in need because you actually care about them; you tell the truth only because you know you’re supervised. I probably don’t believe this about you unless you are certifiably psychopathic.

And the second point, which I think can testify to the first, is that no one who is morally apathetic sans godly provision would, in the first place, use morality as a talking point to convince a non-believer.

That the moral argument is employed persuasively signals that you find morals valuable, and that therefore you are not moral exclusively because of perpetual supervision by a great punisher.

If indeed humans are so wretched, a collection of strictures such as morals would be anathema - entirely unhelpful to your case, like advertising your restaurant by appealing to its customary practice of enforcing the consumption of rotten meat. You are betraying an independent human investment in morals by using them argumentatively.

Another analogy: if I argue that students should be left to their own devices in classrooms, isolated from the invigilation and guidance of a teacher, you might respond that in this case nothing would get done. Few children would actively pursue textbooks or other forms of academic learning.

If you did respond this way, what would it say about you? That you believe learning is valuable? Or that learning is only valuable when a teacher is present?

If you believed the latter, which I think nobody would, then the teacher’s absence should make no difference. The former position suggests value in education whether there is a teacher or not.

If the world is a classroom, I say we students need no such teacher and that we can learn of our own volition.