We can know some things about Jesus for sure. Some things that we have to know to be saved, like that Jesus is the Christ, the Son of God, Lord, Light of the World, that hecame in the flesh, died for our sins, was raised by God the third day, and then was seen by over 500 witnesses.
Some things we know for sure, although they are not necessary for salvation, but they are clearly stated in the Bible. Like that he was the Son of Man, the most important prophet, born of a virgin, and had power to do miracles and make perfect speeches.
But some things are unknown, because we are not supposed to know, see Luke 10:22 "No one knows who the Son is except the Father". So even the apostles did not know. They included clearly things like whether he is God, and whether he preexisted.