The church of St. George, in Rizarios, is one of the many small gems, one can discover wandering around Athens. It is located close to center, in the metro station of Evaggelismos. In a close distance you can find both teh Byzantine and Christian museum as well as the war museum.
This is a link on how you can get there. https://bit.ly/2w5YyST
Go on a Sunday morning, early. You will have the opportunity to attend a genuine Greek orthodox mass.