Fraggle,
The problem with your statement is that it's either true of everyone (we all, theists and atheists, believe that what is, is), or it doesn't really say anything at all about anyone.
For instance, it seems like you might be saying: Theists believe that what is, is God. Atheists believe that what is, is [something other than god].
The problem is that not all atheists will agree to what that something other than god is. So basically all you've done is restate that atheists don't believe in god, but in more confusing terms.
Finally, your statement conflates atheists with strong atheists. That is, some atheists will admit that possibility of god, though they don't believe in it. For them the statement might look like this, "What is, is [god or something else, but probably something else].
Which muddies the waters even more.