We do not.

Most atheists hold the position that their prior positions were not right. They are in doubt. Many atheists have "deconverted" from one religion or another, which emerges as an admission that they were wrong about their theistic beliefs.

This question sometimes arises due to a misconception about what an atheist is.

The default position for any claim is to not accept it until it is demonstrated beyond a reasonable doubt. That is not the same thing as believing the opposite of a claim.