The basilica is 38 meters long, 24 meters wide, and 15 meters high. Almost every surface is local stone with a different geographical address. The floor is paved in Pentelic marble, quarried from the same mountain that built the Parthenon. The twelve columns supporting the nave are 5 meters of green marble each, brought across the Aegean from the island of Tinos. The frescoes were a different journey. The semi-dome of the triumphal arch is filled with the Apotheosis of Saint Dionysius the Areopagite, painted in 1890 by the Italian Guglielmo Bilancioni from Rimini, who lived from 1836 to 1907. The four Evangelists stand in life size on the pillars beneath. Christ Pantocrator holds the Gospel above the arch. To the right, Moses with a parchment. To the left, the Prophet David playing a harp. Below them, Saint Gregory the Great and Saint Augustine. None of it is subtle. All of it is paid for.

King Ludwig I of Bavaria, Otto's father, did not stop intervening when his son went south. He commissioned eight stained-glass windows for the church from Carl de Boucher, the director of the royal workshops of Munich, and donated them. Walking down the aisles, you pass a parade of saints chosen for a particular Catholic-Bavarian sensibility. The right aisle holds Saint Amalia, Pope Sixtus II, Pope Telesphorus, and Athanasius of Alexandria. The left aisle holds Otto of Bamberg (the king's namesake), Pope Anterus, Pope Anacletus, and John Chrysostom, the Patriarch of Constantinople. To either side of the sanctuary stand two marble pulpits, donated personally by Emperor Franz Joseph I of Austria when he visited Athens in 1869. A Habsburg gift, in a German-designed church, in a Greek capital, dedicated to the patron saint of Athens. There is no real word for this except Catholic Europe.

On May 14, 1962, Princess Sophia of Greece and Denmark married Infante Juan Carlos of Spain. The Catholic ceremony took place at this basilica, performed first because Juan Carlos was Roman Catholic and Sophia was Greek Orthodox, and only after the Catholic rite did the couple proceed to the Metropolitan Cathedral for the Orthodox ceremony. Sophia had converted to Catholicism before the wedding, a condition required by the Vatican. Juan Carlos, in 1975, would become King of Spain after the death of Franco, and Sophia would become Queen Sofia. The young couple's first marriage rite, the one that actually came first in the order of the day, took place under Bilancioni's apotheosis fresco. Inscriptions on either side of the main entrance, one in Latin, one in modern Greek, mark the second great Catholic moment of the cathedral: the visit of Pope John Paul II in May 2001.

Above the main entrance, on a platform built in 1888 by the architect Paul Sambi (sometimes Paul Chambaut), stands the cathedral's pipe organ, an instrument almost unheard of in the Greek liturgical world. The dedication of the cathedral itself reaches back to the foundation of the Athenian church. Saint Dionysius the Areopagite was a member of the Areopagus, the ancient Athenian council that met on a small hill west of the Acropolis. According to the Acts of the Apostles, when Paul preached in Athens and was brought to the Areopagus to defend his ideas, Dionysius listened and converted. He became, by tradition, the first Bishop of Athens. Two thousand years later, the city's Catholic minority chose him as their patron and built him a basilica in the middle of the modern boulevard named for the university. The bishop converted on a hill in the 1st century has the cathedral on the avenue.