Is this on óne ticket, or spread over two?

In Athens arrivals and departures are separated, meaning that you will need to exit and re-enter to make connections. Furthermore if you are coming in from outside Shengen you might need to pickup and re-check your luggage.

I would say 50 minutes is short if you're not an avid traveler. With Elite status you might be able to skip the queues and make it, but otherwise it can get crowded in Athens, especially during holiday season.