How do you know Jesus was not simply “a good man” and nothing more.

When Christ came to the world, he brought a message never before heard, and that message can be summed up thus “I am the Son of God, I am God”. He never used these exact words but his teachings clearly and unequivocally put forth this message and his power over sin (and other miracles) shows his divine authority. The Apostles believed he was God, and the Jews got this message clearly and this is why they had him killed.