My own religion
More than just not believing but knowing why. An "informed" atheist has studied reasons for and against theism and decided that the pro arguments are lacking.
If you don't know and can't argue against Pascals Wager, the Kalam and Aquinas 5 Proofs, you are not an informed atheist.
To nitpick even what theism is, has some limited wriggle room. I mean I know of at least several humans, which are as theists no different in practice than any other left-wing modern Europeans.