John 15:26-27
“When the Comforter has come, whom I will send unto you from the Father, even the Spirit of Truth, which proceeds from the Father, He shall testify of Me. And ye also shall bear witness, because you have been with Me from the beginning.”
“He shall be worthy of belief, for He is the Spirit of Truth.” On this account He called It not “Holy Spirit,” but “Spirit of Truth.” But the, “proceeds from the Father,” shows that He knows all things exactly, as Christ also says of Himself, that “I know whence come and whither I go”, speaking in that place also concerning truth. “Whom will send.” Behold, it is no longer the Father alone, but the Son also who sends. “And ye too,” He says, “have a right to be believed, who have been with Me, who have not heard from others.” Indeed, the Apostles confidently rely on this circumstance, saying, “We who ate and drank with Him.” And to show that this was not merely said to please, the Spirit bears witness to the words spoken.
Source: Homilies on the Gospel of John (New Advent)