I'd say that not believing in god comes naturally as people are not born religious, they are taugh to be religious. A person's natural tendency, if left to their own devices, is to be atheist. Personally, I would also say that it is more natural to be atheist because it is more logical, but that might just be me.
We are born without a belief in any god or gods; it has to be taught to us.If we were born in Tel Aviv, we might believe in Jehovah;
if in the American Bible Belt, we would be taught about Jesus;
if in Afghanistan, we'd learn about Allah.
(These three are all the same god, of course -- modern extensions of the tribal god of the Hebrews.)
If we were born in China we'd be taught about Buddhism or Taoism;
if in India we might become a Hindu or a Zartushtie (they migrated there from Iran);
if in Central America we might learn about Quetzalcoatl or Catholicism.
Religion was primitive mankind's attempt to explain things they lacked the education to understand. By this 21st century it should have long ago petered out, but it's too profitable an industry to allow it to die so easily.
I don't believe that either come naturally. What influences theistic beliefs is the environment someone is raised in. Raised in a town of pagans=pagan, same with all other religions. Raised in the forest by jaguars, you probably wouldn't think about God.