Errr... wrong.
It does not follow that if there is no god there is no basis for good and evil.
Now, first let me address an objection that I can already see. I do not accept the idea of "absolute" morality, in the same way that I don't accept the idea that "infinity" actually
has an existence here in the real world. It is merely a concept invented by humans to describe "that thing so big it never ends".
However, I do accept the idea of "morality maintained over time", and I find, on questioning, that this is what most christians mean when they talk about absolute morality. They speak of things such as "morality breaking down should god be neglected" and similar concepts. They think of "god" as a state machine.They think that without the nebulous concept of a "god" that morality has no way of
being maintained over time.
This, unfortunately for them, does not follow.
All that's required to knock over the argument is to demonstrate an alternative method of maintaining "morality" over time. And we have that.
We have society, and we have the culture that comes along with society. It's the way we maintain all of our knowledge over time, morality included.
This is amply demonstrated in the demonstrable differences between cultures. Cultures vary between themselves, but nevertheless those cultures have persisted over time, with near-identical moral values from one generation to the next. This, by definition, is the maintenance of morality from generation to generation, and it does not require a particular god. Or, in fact, any god at all.
Maybe there
is a god. I've seen no evidence of it, but maybe, possibly. It's a long shot, very long. But perhaps. However an alternative exists for the maintenance of morality over time, and that is society and culture. The very existence of this alternative is enough to render @CreationPhysics's argument null. The fact that there is evidence for this explanation and no evidence for the "god" version takes it a step further and adds a piquancy to the theists' defeat. We know society and culture exist, and we know they can do the job. We don't know "god" exists, and as a result we're merely
positing that it can do the job.
Social continuity wins. God loses.
God is not required. God is, in a word, extraneous. And if it is extraneous, it is capable of being a poison in the chalice of reality. And so it is.
Checkmate.