Most religions (I use this word in its broadest sense) have a primary witness. Someone who claims to have seen or heard from God, or experienced a unique awareness of some force, power, or knowledge. The testimony of this witness usually becomes the foundation of the religion.
Christianity is multi-tiered in its cadre of witnesses. There are those who testify in the Bible that God spoke directly to or through them. Then there are those who, throughout history, have confessed to a definite, real, experience with God. I am in that group... I have experienced God.
But Jesus is the primary witness, and He is also the transcendent witness. By this I mean He is far past simply being a conduit of God's revelation or message. He is the revelation. He is the message. The fullness of His knowledge, being, and purpose transcends this existence. But He came to us... The Apostle John seems to have understood this reality better than most, so this insight is boldly woven into his writings.
This is the chief realism of Christ followers--Jesus is Emmanuel--God with us. He is the primary witness, and there is none like Him.