Keep in mind that we're not talking about the truth of any mundane matter, but the very nature of the almighty God, only God himself has a say on that, for He alone knows himself, anything we've heard and learned are secondhand information proceeded from human sources, they're just hearsay. Other truths in life can be known based on personal experience and physical evidence, but the truth of God is on a whole new level. So you're right to a certain extent, that truth "cannot be known" - unless it has been revealed to you by God himself. And please note that Jesus didn't ask "what does the Scripture say I am," knowing the truth of God requires a personal encounter with Jesus like Saul of Tartus being knocked off horseback, reading and studying him from the bible is not enough, that only enables you to know ABOUT God, but not knowing him closely and intimately.
When Jesus came into the region of Caesarea Philippi, He asked His disciples, saying, “Who do men say that I, the Son of Man, am? So they said, “Some say John the Baptist, some Elijah, and others Jeremiah or one of the prophets. He said to them, “But who do you say that I am?”Simon Peter answered and said, “You are the Christ, the Son of the living God.”Jesus answered and said to him, “Blessed are you, Simon Bar-Jonah, for flesh and blood has not revealed this to you, but My Father who is in heaven." (Matt. 16:13-17)
Which one? The one that comes in the flesh. Which one is yours?
Beloved, do not believe every spirit, but test the spirits, whether they are of God; because many false prophets have gone out into the world. By this you know the Spirit of God: Every spirit that confesses that Jesus Christ has come in the flesh is of God, and every spirit that does not confess that Jesus Christ has come in the flesh is not of God. And this is the spirit of the Antichrist, which you have heard was coming, and is now already in the world. (1 Jn. 4:1-3)