You can't be agnostic if you believe that a god exists, I think that's called a retard.
Of course you can. Agnosticism is a conviction about provability. Belief in god(s), is usually characterized by it's lack of proof. Indeed, that's what makes faith different from mundane convictions based on proof.
So you can be an agnostic and a true believer. I have a good friend who claims both positions. In fact, most religious people I've talked to will make agnostic claims, or at least admit that they can't prove the existence of gods.
And, fwiw, you can also be an atheist and an agnostic.
FYI, from Wiki, "an agnostic is someone who neither believes nor disbelieves in the existence of a deity or deities, whereas a theist and an atheist believe and disbelieve, respectively".
Agnosticism - Wikipedia, the free encyclopedia
Heh... the first line of the opening paragraph pretty much makes my point. Did you read it?
Agnosticism is the view that the truth values of certain claimsespecially claims about the existence or non-existence of any deity, but also other religious and metaphysical claimsare unknown and (so far as can be judged) unknowable.[1][2][3] Agnosticism can be defined in various ways, and is sometimes used to indicate doubt or a skeptical approach to questions. In some senses, agnosticism is a stance about the difference between belief and knowledge, rather than about any specific claim or belief. In the popular sense, an agnostic is someone who neither believes nor disbelieves in the existence of a deity or deities, whereas a theist and an atheist believe and disbelieve, respectively.[2] In the strict sense, however, agnosticism is the view that humanity does not currently possess the requisite knowledge and/or reason to provide sufficient rational grounds to justify the belief that deities either do or do not exist.
Perhaps I should have prefaced my comments with 'in the strict sense'. Popular usage mangles many useful words, but usually - as it does in this case - also renders them incoherent. If what we mean to say is "unsure", why bother with a fancy word? Just say "unsure".