From what I have found in life, there is only one God.
To me, ones acceptance of One God and the knowledge as to how that One God communicates with humanity, is the answer to all the questions.
Thus the answer to your question is that Baha'u'llah has a twofold station. In one station he was a man like us, in the other station Baha'u'llah is the 'Self of God'.
In the human station Baha'u'llah as a child had some rudimentary education and instruction, one subject would have been the Quran.
In the station of the Messenger, Baha'u'llah's knowledge was innate, unlearnt. These qualities become apparent in the person of a Messenger from birth.
Thus from an infant, people already know there is something very special about these children.
There are many stories recorded about the childhood of the Bab and Baha'u'llah. In the case of the Bab there is much material available to show how the station of the Messenger is untaught knowledge.
I have taken a little time to answer your question, but it requires much study, as we do not have innate knowledge. Baha'u'llah could answer all questions, comprehensively and with bonus input. Thus Baha'u'llah gave knowledge of the Spirit not previously taught.
This brings us a full circle, if you do not believe in God, and thus do not believe we are from the Holy Spirit, then you will reject all this and it will come back to the one same question.
Do you believe in God?
Regards Tony