What does the following lines mean, Sophocles long ago Heard it on the A gaean, and it brought Into his mind the turbid ebb and flow

Sophocles was a Greek writer who wrote plays that explored the human condition. It would seem that Arnold is comparing his speaker to Sophocles as both of them have looked out to sea (he imagines) and have pondered the nature of faith. In this poem faith is compared to a withdrawing sea as Arnold is concerned about the threat to religion from science. As Sophocles lived until around 90 he saw many wars and changes of fortune and thought about the general misery of human existence.