Atheism is often called a "belief" or "religion" by those wishing to drag us down to their level, But nothing could be further from the truth. Atheism is the default state of all human beings. It takes no action on the part of the person to be Atheist. Atheist is what we would all be if there were no vile old men moulding childrens minds to suite their agenda, or moronic parents inflicting their own stupidity on their children.
A child who has not been subjected to the abuse that is religious indoctrination would be an Atheist. They wouldn't all of a sudden decide that there is a god and that 2000 years ago he sent his son to live as a human, have him murdered for sins we haven't yet committed, Bring him back to life and take him back to heaven unless they are told it by someone.
But a lot of people do end up believing exactly that. Do they believe this because of reason or evidence? No, They believe it because that is what they have been told. If as an adult they stumbled across the bible in a book shop would they come to the conclusion after reading it that it makes sense and must be true if they hadn't already been told from birth that they are a christian, or told that they live in a christian nation? Highly unlikely, Unless they are mentally ill, In which case they may also believe they are Admiral Nelson.
Hopefully now you will see how theism requires not only a belief in a god, But it also requires a society or parents to convince you of a specific god and it needs to convince you that they know what he wants.
Nobody needs to be convinced to be an Atheist. A disbelief in a god is a natural as a disbelief in any mythical monster. Theism however requires you to be convinced of something you have been told.