If you are atheistic, you don't believe in God. Or in any gods for that matter. You’re also probably not interested in praying or participating in religious rituals.
The French root of atheistic literally translates to “without god,” and that’s how atheistic folks choose to live — with no god talk affecting their decisions. Some people who are atheistic actually get together to discuss their lack of belief, called atheism. Atheistic is an exception to the rule about “i before e except after c,” so watch out for that.