I am one who testifies for myself; my other witness is the Father, who sent me.”
Morris states, “In the light of His claims no other witness is possible. If Jesus really stands in the relationship to God in which he says He does, then no mere man is in a position to bear witness. No human witness can authenticate a divine relationship.” (443) Observe What 1 John 5:9-12 says:
1 John 5:9-12 We accept man’s testimony, but God’s testimony is greater because it is the testimony of God, which he has given about his Son. Anyone who believes in the Son of God has this testimony in his heart. Anyone who does not believe God has made him out to be a liar, because he has 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. He who has the Son has life; he who does not have the Son of God does not have life.
Note that the Father and Son are two distinct Persons because they give separate testimonies. At the same time, they are one in their testimony, power, and glory.