I believe that Jesus was a great moral teacher. Why do I have to believe that he was God?
Morality was indeed an important part of Jesus' teaching. But it's not the only thing He taught. He also claimed to be God in human flesh, and that no one could come to God except through Him (John 14:6).
In his classic book Mere Christianity, famed British author CS Lewis wrote:
"A man who was merely a man and said the sort of things Jesus said would not be a great moral teacher. He would be either a lunatic--on a level with the man who says he is a poached egg--or else he would be the Devil of Hell. You must make your choice. Either this man was, and is, the Son of God: or else a madman or something worse. You can shut Him up for a fool, you can spit at Him and kill Him as a demon; or you can fall at His feet and call Him Lord and God. But let us not come with any patronising nonsense about His being a great human teacher. He has not left that open to us. He did not intend to."
The claims of Jesus as recorded in the Bible draw a definite "line in the sand," and put all of us on one side or the other.