But let's look up the words in a dictionary for a second.
AtheistPersonally, I believe there is no God, but I fully acknowledge that this cannot be known with certainty. I am, then, both atheist and agnostic. Similarly, it would indeed be possible to be both Christian and agnostic. In fact, everyone who are not insane should admit that whatever they believe about deities, we cannot ever know for sure. Thus, everyone should be an agnostic. The insane would then include all those who claim to have proof positive of their favorite fairy, say through revelation (but exclude those who are lying about that, like the Pope).
Webster: One who believes that there is no deity
Oxford: One who denies or disbelieves the existence of a God.
Webster: A person who holds the view that any ultimate reality (as God) is unknown and probably unknowable ; broadly : one who is not committed to believing in either the existence or the nonexistence of God or a god
Oxford: One who holds that the existence of anything beyond and behind material phenomena is unknown and (so far as can be judged) unknowable, and especially that a First Cause and an unseen world are subjects of which we know nothing.
Tom Rees wrote a post recently about the definition of atheism, with the take-home message that we should define explicitly what it is that we disbelieve. With respect to the God of the Bible, there is the problem that there are many interpretations of who that dude is. Yet, if you tell me anything specific about that God, then I definitely will say that I do not believe in that (given that you'd say any of the usual things, that is). But in addition to my belief of no God like that, there is also the issue with evidence. While it is possible to posit a deity that one cannot find any evidence against (e.g. Deism), it is not possible to believe in very much of the Bible without admitting that some of those claims is up for scientific inquiry. Genesis 1, for example, which explains how species were created in their present form (i.e. phenotype, for those of you who aren't theologians). I guess that's why so many Christians have a huge problem with evolution. That, and that they are animalophobes.