Where does the trend of having a diploma come from? Did ancient Greek teachers have anything like a diploma to certify their knowledge?
The word comes from Ancient Greek διπλόω = di (double) + ploo (fold). In ancient times, important documents (like this military diploma granting citizenship after the term of service) were engraved on diptychs folded one over the other to preserve the inscription.
In the educational context, I suspect you’ll find diplomas or similar documents were issued pretty much as soon as educational or examining institutiions emerged, awarding qualifications to which anybody attached any value. If you’d met the requirements of the Wood-Carvers Guild to be recognised as a Master Wood-Carver, you’d want to be able to evidence that, wouldn’t you? So the Guild would give you a certificate of some kind.
I don’t think that that is obvious at all. More to the point, it clearly wasn’t obvious to many of those institutions at the time.
It has only been in the past few centuries that it has become the norm for educational and professional bodies to issue such certificates. Medieval universities, guilds, corporations etc. usually didn’t. Yes, they might issue licences for specific purposes and, yes, they would happily provide written certified extracts from their records if required. But often the only written records of admission or qualifications were ones the organisation kept in its own archives.
The check against fraudulent qualifications wasn’t that an individual had a bit of paper, but rather the assurances of trusted persons in a position to know whether that individual was who they claimed they were. This was usually just as effective.