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...