In Who's Afraid of the Big Bad Worldview?, I mentioned the committed agnostics who are unsure of God's existence. I address the problems of agnosticism here.
I want to be clear that this discussion does not concern those individuals who at present, are uncertain of whether or not they believe in God, but anticipate resolving the matter in the future. This stage is necessary for many people and I wish them all the best in sorting the matter out. No, I am addressing agnostics who claim not only that they do not know currently whether or not God exists, but that there is and will always be of insufficient warrant to decide the question.
In my view, there is good reason to be agnostic on many issues. There are questions that I cannot answer and will probably not be able to answer within my lifetime, but that I have no reason to form an opinion about. For example, I am agnostic with respect to the existence of extraterrestrial life, because I think that our knowledge of the cosmos and of the processes by which life forms will for many years be very limited, and because I do not have a compelling reason take a stance on the issue. (A few people would argue that I do have good reasons to take a stance, but their suggested reasons are almost certainly false, i.e. alien attack is imminent.) There are also issues on which I am currently agnostic and are of practical import, but which I believe I will later have the basis to answer. I don't yet know which professor in my academic department would make the best PhD advisor, but I believe that I will be able to decide the question after a few more weeks of meetings and observation. In general I think that it is acceptable to be agnostic about questions that have little practical significance and good to be agnostic about non-pressing practical questions that one will be able to answer wisely in the future.
The question of God, however, is one of great practical import, and one that new evidence or argument is unlikely to illuminate. The question is ancient and the new arguments pro and contra that arise in philosophical journals are almost always refinements of old ones. If there can be sufficient pre-Apocalytic evidence to decide the question, it is here already. And it is a matter of great practical importance, both in its eternal implications and because to become a Christian, or Jew, or Muslim is not merely to assent to a set of beliefs, but to embrace a way of life. The grim reality is that even though agnosticism is logically sound (one can demand more evidence for anything), and philosophically distinct from atheism, it fails to break from atheism in its practical implications, from atheism's failure to recommend a way of life. Lived atheism and lived agnosticism are the same thing. And if the evidence and arguments surrounding theism are so inconclusive, if there really seems to be a significant possibility that theism is correct, then why should one live as though the question has been decided the other way?