I’ve read that nobody believed in the Greek polytheistic religions by the 9th century AD.
Today, an estimated 90% of Greeks belong to the Greek Orthodox Church.
But at what point in history did the tables turn in favor of Christianity over the old religion(s)?
Well, the most famous turning point is the edict by emperor Constantine (“the great”, Actually Flavius Valerius Constantinus). He granted freedom of religion in 313 AD and the F irst Council of Nicaea happened in 325 AD adding further attention to the Christian religion which soon became the dominant religion in the Imperium Romanum of which Greece was part. (See Carroll, Warren, The Building of Christendom, 1987 among many others). Constantine also moved the capital to Bytantium - essentially starting a process that turned the Roman Empire into a mostly Greek empire in the East.
Until the end of the 4th century Greece had been widely Christianised.
Please note that Greece had been an early hotbed of Christian missionary activities - including Paul (see here https://leibnizsozietaet.de/wp-content/uploads/2012/10/04_irmscher_j.pdf). Also note the letters to the Corinthians and Ephesians (also a Greek settlement at the time) as examples for the importance of early Christian missions. So Christianity had centuries to plant its roots and then, when official recognition came, Greece was quite fast to be mostly Christian.