“But when the Helper comes, whom I shall send to you from the Father, the Spirit of truth who proceeds from the Father, He will testify of Me. 27 And you also will bear witness, because you have been with Me from the beginning.” — John 15:26, 27.
Jesus says that the Holy Spirit will come as the Helper, whom He will send from the Father. He is the Spirit of truth coming from the Father and testifying of Jesus. Jesus’ disciples will also witness of Him, aided by the triune God, because they have been with Jesus from the beginning of His ministry.