John 5:32

“There is another who bears witness to me, and I know that the witness that he witnesses of me is true.”

He does not need to bear witness concerning Himself because there is Another Who bears witness to Him, Someone Whose witness is undoubted. As He will demonstrate later, God bears witness to Him, indeed has already borne witness to Him through the miracle of the lame man. This is the testimony which really counts. But prior to this He will point to an earthly witness.