Jesus had just claimed to be the Light of the World (John 8:12).  But the religious leaders challenged His personal testimony: who could prove that He was telling the truth?  They assumed that He was a liar, and told Him so.  But Jesus exposed their shallow thinking.  A single eye-witness is not false if he or she tells the truth.  Jesus said that He was an eye-witness of heaven; that is where He had come from and the place to which He would return.  He knew Father God intimately.  But His hearers had no such experience and were not qualified to judge His testimony.
 
However, Jesus was not a sole witness to the truth of God; the Father also knew the truth.  And the Son of God was not alone because He represented the Father completely, and the Father was with Him; Jesus was one with the Father (John 10:30).  Everything which came out of the mouth of God the Son was already in the heart of God the Father.  The Pharisees said they were looking for two witnesses, but the testimony of the Father and the Son were presented together through the words and actions of Jesus. Nobody knows God better than God!  He is self-authenticating.  That is why we can never find the truth about God by reasoned deductions from what we can see: we need Him to reveal Himself - which was exactly what He did when Jesus came to earth (Hebrews 1:1-3).
 
God is so much greater than our imagination that we can never find Him except He reveals Himself to us. Think about it; how else could God fully reveal Himself, except by Himself?! (Matthew 11:27).  That is why Jesus came. As Hebrews 1:1-3 says, "In the past God spoke to our ancestors through the prophets at many times and in various ways, but in these last days he has spoken to us by his Son, whom he appointed heir of all things, and through whom also he made the universe. The Son is the radiance of God's glory and the exact representation of his being, sustaining all things by his powerful word. After he had provided purification for sins, he sat down at the right hand of the Majesty in heaven."

That is the reason why true gospel evangelism must be focussed on explaining the character and work of Jesus.    Without that, you are missing the core of God's self-revelation.  Let us not be content with religious community activity, or some belief-system which sidelines the centrality of the Lord Jesus Christ (Galatians 1:6-9); let us get back to announcing Jesus!  If He was bold in saying that there is no other way to God (John 14:6), we should be bold also (Acts 4:29).