Heh heh heh heh heh. Atheism isn't really something that you have to join or convert to. The Sunday afternoon baby-roasts are just a social event, not an actual joining ritual.
Default atheism is sort of a middle ground, compatible with agnosticism. Agnosticism and atheism just address two different questions.
Do you hold an active belief in a god? If you answer, "No," then you're an atheist.
Do you claim to actively know that there's no god? If you answer, "No," then you're an agnostic.
Theism/atheism deals with the state of your beliefs. Gnosticism/agnosticism deals with your claim to knowledge in regards to your beliefs.
If you don't hold an active belief in a god, but you don't actively believe that there isn't a god, then you're an agnostic atheist. Most atheists are.
Look at the prefix this way. On the subject of morality, something can be moral, amoral, or immoral. Amoral is the middle ground, being neither moral nor immoral. Atheism is the same.
It gets a little messy, since we don't have a specific word like imtheism. So, they also get lumped in with atheists, since they're not theists. There are terms like anti-theism, but that tends to be used to refer to people who think that religion is a bad thing and that we as a species need to purge ourselves of that sort of irrational epistemology.
Generally, we tend to differentiate between the different varieties of atheism with a modifier: soft and strong atheism, default and explicit, or something along those lines.
Default and explicit atheism can also be context-dependent. Do I know that there is nothing anywhere in the universe that someone somewhere would call a god? No. But I have no reason to think that there's anything of the sort.
Could there be some sort of trickster god who gets its kicks fucking around with humanity and misleading us, while keeping itself hidden from our knowledge? I suppose so, although if someone actually believes that there is something of the sort, then that person is in the tinfoil-hat category, along with 9/11 Truthers and similar.
Do I know that the Bible is completely full of shit, though? Yes.
What we have in the New Testament is a collection of 4 anonymous testimonies, written decades after the supposed events, three of which are cribbed from some earlier source. We have absolutely no contemporary, extra-biblical confirmation of anything stated in the gospels.
Most of the rest of the New Testament is comprised of a collection of letters, written by a guy who never even met Jesus, by his own testimony. But it's even worse than that, since most modern biblical scholars agree that almost half of those letters weren't even written by Saul/Paul.
And Christians want us to believe that this is how a supposedly omniscient, omnipotent, omnibenevolent being, who wants us to know about him and love him, has chosen to communicate with us? It's freaking preposterous.
The Book of Mormon is even worse, being written by a known con-artist and being full of the most insane unhistorical nonsense that I've ever heard. I've listened to a couple of podcasts done by people who are reading the Book of Mormon, chapter-by-chapter. It's just so ... I don't even know what to call it.
Anyway, get involved in the discussions, around and about, or start up a discussion in a group that would be appropriate for the sort of discussions you want to have or the sorts of questions you want to ask. We have a lot of people who love to discuss the subject of religion, myself included.