This article is from the Atheism FAQ, by mathew firstname.lastname@example.org with numerous contributions by others.
"I know from personal experience and prayer that God exists."
Just as many theists have personal evidence that the being they
worship exists, so many atheists have personal evidence that such
beings do not exist. That evidence varies from person to person.
Furthermore, without wishing to dismiss your evidence out of hand,
many people have claimed all kinds of unlikely things -- that they
have been abducted by UFOs, visited by the ghost of Elvis, and so on.