You can define good as you want, no need to have a religion define it for you.
I believe the main characteristic that takes apart atheist from other belief systems is that ultimately without a god that objectively defines what is good, one must define by himself what is good. This one one side is liberating because there is no more pressure between what one thinks is right and what the religion tell them, but at the same time extremely difficult because one exposes themself to all their psychological weaknesses (and to whom can manipulate them).
I believe the main characteristic that takes apart atheist from other belief systems is that ultimately without a god that objectively defines what is good, one must define by himself what is good. This one one side is liberating because there is no more pressure between what one thinks is right and what the religion tell them, but at the same time extremely difficult because one exposes themself to all their psychological weaknesses (and to whom can manipulate them).