I am absolutely certain that god does not exist. As certain as I am that the world in not flat and that pigs cant fly. It works like this. The supernatural doesnt exist. God is supernatural so god doesnt exist. If it is a god it is supernatural. If it is not supernatural it is not a god. It really is that simple to know god does not exist.
You said: "A belief is what someone thinks to be true. It can actually be true, or they could be wrong."
You are absolutely correct. It might be true and it might not but if you believe it, you don't know if it is true or not. The theist doesnt say: I dont know if god exists, they say God does exist. This is what they claim. The atheist rejects that claim. The atheist knows there is no god just as surely as the theist knows there is a god. No belief is involved here.
The agnostic doesnt know. Not because the agnostic has a belief one way or the other but because the agnostic doesnt have enough knowledge/information or because they believe that the existence or nonexistence of god is unknowable. If you dont know if god exist or not why would you belief it one way or the other? Dont know simply means: dont know
Beliefs are like opinions and dont mean squat. A person can believe anything. There are no limits. Theist do not fly planes into building over something they believe in (dont know). They are 100% certain to take such extreme measures.