Reconstruction of the Acropolis and Areus Pagus in Athens by Leo von Klenze, 1846. via Wikimedia Commons
NOTE: The colossal statue visible on the Acropolis is the Athena Promachos.
The Parthenon was built in the 5th century B.C.E., and remained in use as a temple and treasury into the 5th century C.E.
The Theodosian decrees of 389-391 banned blood sacrifices and ordered temples to be closed -but as in the present day, laws on the books didn’t uniformly translate into changes in human behavior. In fact, evidence suggests the farther from Rome and Constantinople, the less likely Theodosius’ decrees were to be obeyed. Repairs were made to the Parthenon after the building suffered substantial damage, either from invasion or an accidental fire, in the late 4th to early 5th century. An honorific statue of the Prefect Herculius, who served from 408-410, was erected on the Acropolis next to the statue of Athena Promachos to commemorate his rebuilding of the walls of of Athens. An inscription that may be as late as the early 5th century relates that the Neoplatonic philosopher Plutarch donated funds for a float in the Panathenaic Procession three times. A biography of the Neoplatonist philosopher Proclus by his student Marinus states that when Proclus visited the Parthenon in 432, the doorkeeper told him the Parthenon was scheduled to be closed in the near future.
Because of the tenacity of Athenian pagans in the face of Christian persecution, Christians did not become a majority in Athens until the 5th century. By that time, the fervor of conversion had moderated to the point that religious images from antiquity were being reinterpreted as allegory. Such exposition led to the preservation of the sculptures on the exterior of the Parthenon when it was converted to a Christian basilica.
According to archaeologist and historian Bryan Ward Perkins, “An ancient Greek, transported forward in time, who visited the cathedral of medieval Athens, would have had no trouble recognising it as the Temple of Athena.” Quote from: “Re-using the Architectural Legacy of the Past”, The Idea and the Ideal of the Town Between Late Antiquity and Early Middle Ages, ed. G. Brogioli and B. Ward Perkins (Leiden, Boston and Cologne, 1999)