How do you use the word agnostic?

I had always thought that the definition was someone who was undecided about the existence of God. I was speaking to someone who said that she thought it meant someone who believed that the existence of God could not be proved or disproved. I checked a dictionary and found that both definitions were given, with hers listed first.

Using this other other definition, a person could be both an agnostic and a believer, claiming that belief was a matter of faith and was beyond science. My guess would be that this is the older definition of agnostic. Kant was the first philosopher to say that religion was beyond proof, and he was himself a believer. According to the dictionary, the use of agnostic was about a half century after Kant.