Atheistic is to atheist as agnostic is to _____?

http://www.merriam-webster.com/dictionary/agnostic

The word can be either an adjective or a noun.

As a modifier of theist or Atheist, it means non-committed or undogmatic. You can't be a theist and thus believe God exists and at the same time believe it can't be known if God exists. Just the same, you can't be an Atheist and assert no deity exists and at the same time assert that it can't be known whether deity exists.

The agnostic either has the affirmative belief that the existence of deity cannot be known, or that deity exists but cannot be known on a personal level by humans.

The belief that one can have a personal relationship with God is the opposite of the agnostic's belief. Thus the agnostic belief is directly opposed to both theism and deism.

agnostic is the same in noun form as in the adjectival.

Re one answer: theist believes there is a god, atheist believes there isn't. Agnostics don't believe either - the word derives from Greek "not knowing".

Agnostic is both an adjective and noun.

an agnostic

And "atheist" and "atheistic" can mean the same thing if you are using "atheist" as an adjective.

Agnostic is an adjective when used properly to describe either theists or atheists.