TRUTH John 15:26

"When the Advocate comes, whom I will send to you from the Father--the Spirit of truth who goes out from the Father--he will testify about me."

When the Advocate comes 
John 14:16-17a  "And I will ask the Father, and he will give you another advocate to help you and be with you forever--the Spirit of truth."

whom I will send to you from the Father 
John 14:26  "But the Advocate, the Holy Spirit, whom the Father will send in my name, will teach you all things and will remind you of everything I have said to you."
John 16:7  "But very truly I tell you, it is for your good that I am going away. Unless I go away, the Advocate will not come to you; but if I go, I will send him to you."

the Spirit of truth who goes out from the Father 
John 14:17  "...the Spirit of truth. The world cannot accept him because it neither sees him nor knows him. But you know him, for he lives with you and will be in you."

he will testify about me 
1 John 5:6-12  This is the one who came by water (baptism) and blood (death). He did not come by water only, but by water and blood. And it is the Spirit who testifies, because the Spirit is the truth. For there are three that testify: the Spirit, the water and the blood; and the three are in agreement. We accept human testimony, but God's testimony is greater because it is the testimony of God, which he has given about his Son. Whoever believes in the son of God accepts this testimony. Whoever does not believe God has made him out to be a liar, because they have not believed the testimony God has given about his Son. And this is the testimony: God has given us eternal life, and this life is in his Son. Whoever has the Son has life; whoever does not have the Son of God does not have life.

(parentheses are mine)

Scriptures taken from the Holy Bible, New International Version® NIV®, Copyright© 1973, 1978, 1984, 2011 by Biblica, Inc.® Used by permission. All rights reserved worldwide.