Here is an excerpt from St. Augustine's homily for the Gospel reading of the Third Sunday of Advent. This is from the Ignatius Press book "The Sunday Sermons of the Great Fathers" (page 59). St. Augustine is speaking about the important Jewish priests and Levites who were sent to determine the religious identity of John the Baptist:
And they asked him: 'What then? Art thou Elias?' For they knew that Elias was to proceed Christ. To no one among the Jews was the name of the Christ unknown. They did not believe this man was the Christ; nor did they entirely believe that Christ was not to come, since they were hoping He would come. And so they stumbled on Him, present among them; they stumbled as upon an unnoticed stone. That stone till now was small, though already cut out, without labor of hands, from the mountain; as the prophet Daniel says that he is a 'stone cut out of the mountain without hands.' Then what followed? 'And it grew and became a great mountain and filled the whole earth' (Dan. ii. 35).