John 14:26
“The Comforter, whom the Father shall send in My Name, He shall teach you.”
“Perhaps these things are not clear to you now, but 'He' is a clear teacher of them.” And the, “remains with you”, is the expression of One implying that Himself will depart. Then that they may not be grieved, He says, that as long as He should remain with them and the Spirit should not come, they would be unable to comprehend anything great or sublime. And this He said to prepare them to bear nobly His departure, as that which was to be the cause of great blessings to them. He continually calls Him “Comforter,” because of the afflictions which then possessed them. And since even after hearing these things they were troubled, when they thought of the sorrows, the wars, His departure, see how He calms them again by saying,
Source: Homilies on the Gospel of John (New Advent)