He rejoiced in the Holy Ghost, that is, in the works and miracles wrought by means of the Holy Ghost. For He had appointed the twelve disciples, whom He also called apostles, and after them again seventy others, whom He sent as His forerunners to go before Him unto every village and city of Judaea, preaching Him, and the things concerning Him. And He sent them, nobly adorned with apostolic dignities, and distinguished by the operation of the grace of the Holy Ghost. “For He gave them power over unclean spirits to cast them out.” They then, having wrought many miracles, returned saying, “Lord, even the devils are subject unto us in Thy Name.”
And therefore as I have already said, well knowing that those who had been sent by Him had benefited many, and that above all others, they had themselves learned by experience His glory, He was full of joy, or rather of exultation. For being good and loving unto men, and wishing that all should be saved, He found His cause of rejoicing in the conversion of those that were in error, in the enlightenment of those that were in darkness, and in the answer of the understanding to the acknowledgment of His glory, of those who had been without knowledge and without instruction.
St. Cyril of Alexandria, Commentary on Luke, Homily 65, p. 277