All you have to do is redefine good to mean whatever you want it to mean and we can be plenty good without God. But is there even such a thing as good without God?
Here’s an experiment I would recommend to you. Go to a secular setting (college, public school, business, etc.) and participate in a decision making forum or fora. When a business decides what should be done to increase market share, offer a suggestion rooted entirely in love of neighbor as an actual, concrete person. See what reaction you get.
When a teacher’s union decides how to approach the state over pay and benefits, suggest that they should take up their crosses and offer themselves on the alter for the children. Note the response.
Do you need more examples?
Then go to a monastery or a convent or a prayer meeting or a missionary gathering.
I am only too well aware of the hypocrisy we Christians show because we fail to live up to our callings. And I’m also aware of things like the Peace Corps, where young idealists borrow from Christian ethics to make the world a better place. So I recognize that this is not a clean cut empirical experiment.
But you can’t love your neighbor as though He is the Image of a God you don’t believe in. And to think of Him as less than that, and to think of yourself as less than that, is in itself already immoral and is the foundation of every immoral act.
That it is so hard to be good with God hardly argues that we can be good without God.