Q. When did Jesus know He was God the Son? The Second person of the Trinity?
A. He always KNEW who He was. Jesus was a God/man. God and Man joined hypostatically. So He, the God/Man in His totality knew he was God. One of the attributes of God is Omniscience so it would be impossible to be God and not be omniscient. His divine nature always knew He was divine, even as an embryo in Mary’s womb.
Jesus had two natures, however, Human and Divine. When Jesus said that He didn’t know the time of His return He was speaking
“in virtue of His assumed human nature. This knowledge was proper to Him as God, but not proper to Him as man. He did not know God’s moment in virtue of the human nature in which He had come to teach mankind, and He merely brought out the fact that this particular piiece of information was not part of His message…It was God’s secret. Radio Replies Vol II p. 153-154
In other words, Jesus spoke from his human knowledge only, but at the same time His Divine Nature certainly did have knowledge of His return. And this information could have been revealed to His disciples if He had wanted to. But He did not.
The same is true when in the Gospel, after Mary and Joseph find Jesus in the temple, it says Jesus “grew in wisdom and stature” Lk 2:52
Again, it was His human nature/body that “grew in wisdom/knowledge and height”.
Since He was truly human His human nature had to learn things in a human way despite the fact that His Divine nature could have infused knowledge into His human nature. He experienced the fullness of humanity.
Before I became Catholic, on my own, I concluded that Jesus became truly divine at His baptism or at the least that is when He knew Who He really was–God the Son. This is the heresy of Adoptionism- an 8th century Spanish error. So, our Enemy is still up to his old tricks.