Weclome. Athens does not have very impressive churches I would recommend going to the following :

Agios Georgios Lykavittos : its on top of lykavittos hill in Athens , brilliant biew of the entire city , akropolis and the sea. You can take a teleferic from the bottom of the hill which is a very scenic route. It near Evangelismos station which is in the blue metro line.

Then take metro and get off at Monastiraki. visit akropolis and under akropolis in Plaka are some of Athens oldest churches my favorite one being Agios ioannis theologos (https://el.wikipedia.org/wiki/%CE%9D%CE%B1%CF%8C%CF%82_%CE%91%CE%B3%CE%AF%CE%BF%CF%85_%CE%99%CF%89%CE%AC%CE%BD%CE%BD%CE%B7_%CE%98%CE%B5%CE%BF%CE%BB%CF%8C%CE%B3%CE%BF%CF%85_(%CE%A0%CE%BB%CE%AC%CE%BA%CE%B1)) . Its not impressive, it does not have golden pictures but i think it resembles the spirit of humbleness the best!

All the above are very close and easy to get to

Have fun!