The term "agnosticism" itself was coined by Professor T.H. Huxley at a meeting of the Metaphysical Society in 1876. For Huxley, agnosticism was a position which rejected the knowledge claims of both "strong" atheism and traditional theism. More importantly, though, agnosticism for him was a method of doing things.
Thomas Henry Huxley (1825-1895) was an English natural scientist and author who became widely known as "Darwin's Bulldog" because of his fierce and uncompromising defense of Darwin's theory of evolution and natural selection. Huxley's career as a public defender of evolution and antagonist of religion began most fully when he stood in for Darwin at a 1860 meeting in Oxford of the British Association.
At this meeting, he debated Bishop Samuel Wilberforce, a cleric who had been attacking evolution and naturalistic explanations of life because they degraded religion and human dignity. Huxley's counterattacks, however, made him very popular and quite famous, leading to many speaking invitations and many published articles and pamphlets.
Huxley would later become famous again for coining the term agnosticism. In 1889 he wrote in Agnosticism:
Agnosticism is not a creed but a method, the essence of which lies in the vigorous application of a single principle ...Positively the principle may be expressed as in matters of intellect, do not pretend conclusions are certain that are not demonstrated or demonstrable.
Huxley also wrote in "Agnosticism and Christianity":
I further say that Agnosticism is not properly described as a "negative" creed, nor indeed as a creed of any kind, except in so far as it expresses absolute faith in the validity of a principle, which is as much ethical as intellectual. This principle may be stated in various ways, but they all amount to this: that it is wrong for a man to say he is certain of the objective truth of a proposition unless he can produce evidence which logically justifies that certainty. That is what agnosticism asserts and, in my opinion, is all that is essential to agnosticism.
The reason why Huxley started using the term agnosticism was because he found so many people talking about things as if they had knowledge on the topic when he, himself, did not:
The one thing in which most of these good people were agreed was the one thing in which I differed from them. They were quite sure they had attained a certain "gnosis" - had, more or less successfully, solved the problem of existence; while I was quite sure I had not, and had a pretty strong conviction that the problem was insoluble.
So I took thought, and invented what I conceived to be the appropriate title of "agnostic." It came into my head as suggestively antithetic to the "gnostic" of Church history, who professed to know so much about the very things of which I was ignorant.
Although the origins of the term agnosticism are normally attributed directly to Huxley's involvement in the Metaphysical Society in 1876, we can in fact find clear evidence of the same principles much earlier in his writings. As early as 1860 he wrote in a letter to Charles Kingsley:
I neither affirm nor deny the immortality of man. I see no reason for believing it, but, on the other hand, I have no means of disproving it. I have no a priori objections to the doctrine. No man who has to deal daily and hourly with nature can trouble himself about a priori difficulties. Give me such evidence as would justify me in believing in anything else, and I will believe that. Why should I not? It is not half so wonderful as the conservation of force or the indestructibility of matter...
It should be noted in all of the above that for Huxley, agnosticism was not a creed or a doctrine or even simply a position on the issue of gods; instead, it was a methodology with respect to how one approaches metaphysical questions generally. It is curious that Huxley felt the need for a word to describe his methodology, for the term rationalism was already being used to describe pretty much the same thing. It is important to keep in mind that while Huxley introduced a new name, he certainly did not introduce the perspective or method which that name described.