One of my favourite U2 songs, Hawkmoon 269, contains the lyric that "faith needs a doubt". You can quite easily make the case that Bono was simply stating the obvious. Faith is not objective knowledge; religion would become spectacularly useless if God revealed himself and his existence, like that of the sun, became commonplace knowledge discernable to anybody on the planet. Faith requires an element of doubt; to believe in God is a willingness to affirm the claim that he exists while implicitly acknowledging that he may not. It is for this reason that I believe agnosticism can be useful for the Christian.
It is imperative that I clarify what I mean by agnosticism. I am not referring to the strand of agnosticism that believes the existence of a deity or deities cannot be proven or disproven. I am referring to the strand that neither affirms nor denies the existence of a deity or deities, but rather views the evidence as insufficient; God is not proven or disproven but there is the possibility that he could be proven or disproven. It is not a claim that God does not exist; it is a claim that "on the basis of the knowledge and evidence I currently possess, I do not know whether or not God exists". It is intellectual honesty.
Agnosticism cherishes the doubt of faith. This doubt, this uncertainty, is - to lift another U2 line, this time from Zooropa - a guiding light. It stimulates a rigorous, enlightening, and fulfilling process of questioning, of seeking answers, of testing answers, of verifying evidence, and so forth. Agnosticism is not an excuse to give up, to throw one's hands in the air and say "that's it!" or to be lazy and state "well, since it can't be known; why bother?" Some agnostics may subscribe to this kind of apatheism and I don't blame them. But agnosticism can also be a prompt to delve further into theology - and into history, sociology, politics, and all else that is connected and seeks to understand the human condition. While faith can often devolve into an acceptance of the current answers with no effort to investigate further, as demonstrates by hordes of Christians content with their middle class Sunday Christianity, doubt inherently challenges the answers. This aspect of doubt is of the highest value in the quest for a deeper, richer grasp of many aspects of life. As the doubt of agnosticism and the doubt required by faith exist on the same continuum, the edifying role of doubt in the agnostic's intellectual life can also be useful and edifying for the believer to delve further into theology. The agnostic perspective on doubt can be used by the Christian to more fully understand faith, to flesh it out and examine its many facets more broadly.
Agnosticism and Christianity are not irreconcilable, opposing forces. The Christian need not look upon the agnostic as a theological opponent, but a theological companion. I am and I hope always will be the same Axver that I was before, dedicated to the acquisition, analysis, and interpretation of knowledge in a search for truth and understanding. Agnosticism does not change that; it propels me, and at an even greater speed with a heightened curiosity. An appreciation of doubt need not be and is not a concession of defeat or confusion or an act of renunciation (or to use a term I find insulting, "backsliding"), but a valuable opportunity for intellectual discovery.