For religious conversion of a province, the MTTH base is 960 months. The game determines many things like this based on Mean Time to Happen, or MTTH. It could happen faster if the RNG goes your way, or longer. So the mean time for it to happen is going to be 100 months. The mouseover is saying 1%, which is per game month. Even with a diviner who has a learning skill of 20+ it is going to take a long time.
With that low moral authority it will take a very long time to convert each county. As such, it can't be reformed like the other pagan religions and so will always have a low moral authority. You have an uphill battle, because Hellenic is a sort of leftover religion Paradox never really intended anyone to play.
