Our Lord didn't live in a vacuum. He had Apostles, whom He chose and sent, to administer His Church (Mt16:18) and teach to the world everything that He wants us all to know, especially about our Maker and about His plans for us.Asked:
God is omniscient.
Jesus told his disciples He did not know the day and hour of His return.
Jesus is not God.
How can Jesus be God if He didn't know what God knew?
This is what this man believes.
What is the best answer to this?
One of the very earliest things that His Apostles taught was about the nature of our Maker, and part of that is that our Maker became a human being, born of a virgin.
The details of the Trinity doctrine have been revealed gradually and more and more deeply through time, as the Apostles' successors the Church's bishops have seen fit to reveal, as circumstances and controversies arose.