Normally it would already take some time, and now covid has seen a lot of suspensions in the area. The best way would be to go via Italy:

• go back to Split
• ferry Split - Ancona (Eurail pass holders get a reduction)
• train Ancona - Bari or Brindisi (IC are without the need for reservation)
• ferry Bari or Brindisi - Patras, Greece
• Rail replacement bus Patras - Kiato, then suburban train Kiato - Athens, total travel time about 3 hours. Bus in Patras leaves from the railway station, not the ferry station

You could in theory travel via Serbia, but that would take at least 3 days in the current situation.

Good luck and enjoy the journey!

Thank you!

So there’s no way to get to Greece just by train? Maybe through Bulgaria or any other country around? 
 

Of course you can! But it's just a but more of a hassle in the current situation in my opinion. 

• train Bar/Podgorica - Belgrade (take the day train for a scenic ride)
• train Belgrade - Nis (Nis is worth the stopover)
• bus Nis - Thessaloniki (not included) (the night train Belgrade - Thessaloniki via Skopje is suspended) (could also try to find a direct bus Belgrade to Thessaloniki) 
• train Thessaloniki - Athens (reservation free of charge)

Or if you want to travel at night in a more roundabout route:

• night train Split - Budapest
• night train Budapest - Bucharest 
• train Bucharest - Sofia 
• bus Sofia - Thessaloniki (not included) (train is suspended)

Can´t say it better as @Mattheas 

Few ferries to Patras start in Ancona (Superfast for example) or even in Venice,

Hi @Mattheas do you know until when the night train from Belgrade-Thessaloniki is suspended? We would like to take the same route (from Split to Athens) around the 10th of September and we would prefer the route via Serbia, because we would love to travel by train from Bar to Belgrade since it is one of the most beautiful train routes of Europe :)

Don't count on the train Belgrade - Thessaloniki this year.

@Anne-Meike as @rvdborgt said dont count on this train as even if it would operate it would just be June - August with luck first September week

Ok, thank you both! And the train from Bar to Belgrade is still operating?