I don't believe anybody really knows what God is. It is just an idea, so how can one person's idea be interpreted when that God wasn't even around physically. It is like telling a blind something he never saw, telling a deft something he never heard, etc.
Even if God is saying - there were so many forms of Gods in this world and whose God should be consider the one? For example the Indian worship cows as their Gods, the Egyptian believe another kind of Gods and the Japanese, Jewish, English, German, French, African, just to name a few.
I think if it is a good message and that person thought that it was so divine it must be from God or a higher spirit then it is something pops up in their mind, subconscious mind, books, songs, news, magazines, TV, movies, etc. Who knows how many forms and shapes of messages we got input into our head every single day and when our head finally digested it we couldn't recall where did we heard it from because it could be a long time ago and that individual thought that it must be God or something, I doubt it.