You see, in the Law, the testimony of two or three witnesses was needed to settle a matter. Nothing could be decided unless you had witnesses.
Jesus was telling these people that He had four witnesses -- more than what was required in the Law.
So He established that His claims were real:
The first witness was John the Baptist. Jesus said, "You have sent to John and he has testified to the truth." By this time, it was common knowledge that John, whose popularity had grown and then had diminished with the arrival of Jesus, had put his stamp of approval on Jesus. He said that Jesus was the Messiah they were waiting for; he called Him the Lamb of God. This was the Man that John had prepared the way for.
Thirdly, Jesus said, " the Father who sent me has testified Himself concerning me." (John 5:37) Remember when John the Baptist and Jesus were in the water? When Jesus was baptized, the heavens opened up and the Spirit of God came down in the form of a dove. The Father declared, "This is my beloved Son, with whom I am well pleased." (Matthew 3:17)
Jesus had a fourth witness, too. He had more than He needed, more than the Law required. "You diligently study the Scriptures because you think that by them you possess eternal life. These are the Scriptures that testify about me." (John 5:39)
Over the course of His life and ministry, Jesus would fulfill over three hundred Old Testament prophecies. No other being, either before Him or after Him, has ever fulfilled even a fraction of these prophecies. These were like Jesus' personal calling cards -- they proved He was who He claimed to be.
Over and over in John's gospel, we will see that Jesus fulfilled or exceeded the Law; He offered more witnesses here than He was required to. He proved that He was who He claimed. Now, that doesn't mean that everyone believed Him.
yet you refuse to come to me to have life.“I do not accept glory from human beings, but I know you. I know that you do not have the love of God in your hearts. I have come in my Father’s name, and you do not accept me; ( John 5:40-43a)How sad that they would not accept Him and His gift of life. How wonderful that some did, and their lives were changed -- as ours can be, when we accept Him!