My proposal for an Atheist Logo:

1 - Since everything either spirals in or out in this Universe, here's the Spiral.

2 - It also represents a Birdsnake, a fantasy hybrid link between reptiles and volatiles. Evolution.

3 - It has an organic sketchy-like irregular rendering, indicating the finest any (known) given star can produce: Life.

4 - Also suggests a galaxy... of like-minded thinkers.