You can believe that Jesus lived on earth because of history, but when you don’t follow Him and do what He says, then you don’t believe all at the same time. You have to get saved and believe in Him to really believe.
Because you can say you are a christian and that You follow God but never put your faith in Him and trust Him so that faith in Him is a bond you are trusting Him to guide your life and show you to the right path in life. So that is why we have to be rooted in Him and trust His word.
God gives us free will, he will never force us to follow him and it is for this reason that you have to have faith in him. God gives us an open invitation to come to him but it is ultimately our choice