The doctrine that the existence of God, or any ultimate cause beyond material phenomenon, cannot be known nor denied. The term is derived from the 'unknown' God in Acts 17:23, and was first used in 1869, by Thomas Henry Huxley. Etymologically, the word is from a-, "without" + gnostic, which is from the Greek gnostikos (γνωστικος), which is from gnosis, "knowledge." In a letter dated March 13, 1881, Richard Holt Hutton wrote that the term "agnostic" was "suggested by Prof. Huxley at a party held previous to the formation of the now defunct Metaphysical Society, at Mr. James Knowles's house on Clapham Common, one evening in 1869, in my hearing. He took it from St. Paul's mention of the altar to 'the Unknown God.'"
"But there is nothing
per se irrational in contending that the credences of
Theism are inconclusive, that its
doctrines are unintelligible, or that it fails to account for the facts of the universe, or is irreconcilable with them. To express this kind of polemic against religious
faith the term 'agnosticism' has been adopted." - E. Conder,
Basis of Faith, 1877
Note that an agnostic is not strictly a person who does not know whether God exists. An agnostic, by definition, does not know whether God exists. However, a person who does not know whether God exists is not necessarily agnostic.
Agnosticism is not "the belief that nothing can really be known without concrete logical proof." It is only the doctrine that it is not possible for a human being to know whether God exists or not.
Were an agnostic supposedly "presented with provable evidence" that there is no God, he would not change his agnostic perspective to disbelieve in God. An agnostic believes that it is impossible for the human mind to know whether there is a God or not. The presented "provable evidence" would in fact be unprovable, because the recondite knowledge of the existence or nonexistence of God would still be considered unknowable by the human mind. While evidence might appear to an individual to be decisive, an agnostic believes that it is impossible for the human mind to know whether the evidence, and reasoning leading to the proof of the nonexistence of God, is in fact sound.
Similarly, were an agnostic supposedly "spoken to by the Christian God," he would not change his agnostic perspective to believe in the Christian God. A God, by definition, is superior to all other beings, possibly with the exception of other Gods. Thus, even a being of seemingly infinite knowledge and power would not necessarily be God. Therefore, were a being of such power to speak to a person, that being might appear to be God, regardless of whether the being is or not, and so could, in fact, not be. In other words, "any sufficiently advanced technology is indistinguishable from magic" (Arthur C. Clarke). Thus, in fitting with agnosticism, that person could not know whether the being they encountered was God or not, and so would not change their belief.
See also:
atheism,
theism