If I testify about myself, my testimony is not valid.
A person can claim to be the smartest person in the world, and it may be true. But it isn’t true just because she/he says so.
In fact, if one is the most intelligent in the world there will be facts to substantiate the claim. Without such verification, it isn’t logical to naively accept whatever a person says.
Here Jesus isn’t saying His testimony about Himself is not true. He is saying the legitimacy of His claims and teachings can not and do not rest on his personal testimony alone.
He doesn’t expect anyone to believe in Him just because He said so. He introduces corroborating evidence for His claims - confirmation in addition to His own testimony –Exhibits A,B,C,& D!
Exhibit A - You have sent to John and he has testified to the truth.
Exhibit B - I have testimony weightier than that of John. For the very work that the Father has given me to finish, and which I am doing, testifies that the Father has sent me.
Exhibit C - And the Father who sent me has Himself testified concerning me.
Exhibit D - You diligently study the Scriptures because you think that by them you possess eternal life. These are the Scriptures that testify about me…
Faith is not based on Jesus’ testimony alone. John the Baptizer, the work of teaching and miracles, the Father, and the prophecy of Scripture all testify to the truthfulness of who He claims to be.
And there’s actually an indictment for those who refuse to follow Jesus - yet you refuse to come to me to have life.
The question is not the validity of Jesus. Rather upon what basis can one legitimately refuse Him in the face of the evidence?