What basis is such a writing even true?
The "basis" that it is true is that it was written by Baha'u'llah, a Manifestation of God who had knowledge from God. He also knew all that Jesus experienced better than the gospel writers since He was the same Spirit of Jesus. Thus at one time He said He longed for the cross.
There are two issues here. What do you understand "Essence" to mean? It sounds like you think no one can even have a clue.
I understand Essence to mean what God is in Essence, which I understand to mean to be a Spirit, which my Jesus taught, "God is Spirit."
You raise a good point. What I mean by Essence is the intrinsic nature of God. I agree that we can know God is Spirit because the Bible says that but we cannot know any more about God's nature than that. By contrast, we can know some of God's Attributes such as
Good, Loving, Gracious, Merciful, Just, Righteous, Forgiving, and Patient because they were reflected in Jesus.
Jesus was a clear mirror, and God became visible in the mirror. This is why Jesus said,
“The Father is in the Son” (John 14:11, John 17:21) meaning that God is visible and manifest in Jesus.
“I and my Father are one” (John 10:30) means that the Manifestation of God, in this case Jesus, and God are one and the same, so whatever pertains to the Manifestation of God, all His acts and doings, as well as whatever He ordains and forbids, is identical with the Will of God Himself.
1 Timothy 3:16 King James Version (KJV)
16 And without controversy great is the mystery of godliness: God was manifest in the flesh, justified in the Spirit, seen of angels, preached unto the Gentiles, believed on in the world, received up into glory.