But that is beside the point. The Hebrew word "amen" does not relate to divinity. It is a common word, used in everyday speech, preserved in such conversations and teachings as "amen, amen, I say to you that..." and he wasn't speaking to or of a god or of anything divine.
It is used commonly, not reserved for cultic speech.
The oldest use that we know of of a word is not necessarily the source. One could ask, for example, where the Egyptians got it. Unfortunately, that seems to be a fruitless search, for the records are not extant, and probably could not be. So we would have to be careful in saying that it is the source. It could be, but we do not know.
There are false etymologies aplenty, based on exactly the kind of thinking that this amen/Amen supposition has in it. "Semite" does not come from the Latin "semi" but form the Hebrew "Shem." Greek has two words spelled identically that mean two different things--"thumos" is "anger," and "thumos" is "lentil." The only difference is the accent. We have words that mean the opposite of themselves: "cleave" can mean to "cut in two" or "cling together as one."
So be careful when tracing the etymology of a word. If there is no relation between two similar-looking words in meaning, then it's most likely that there is no relation between them at all. Amen in Egyptian was a god. Amen in Hebrew is a common word meaning "absolutely," or "truly," or even "me, too," plus the meanings of security and firmness.