Matthew Henry Commentary John 15:26-27

"But when the Comforter is come, whom I will send unto you from the Father, [even] the Spirit of truth, which proceedeth from the Father, he shall bear witness of me: and ye also bear witness, because ye have been with me from the beginning." — John 15:26-27 (ASV)

The blessed Spirit will maintain the cause of Christ in the world, despite the opposition it faces. Believers, taught and encouraged by His influences, would bear testimony to Christ and His salvation.