Nick Gisburne makes a brilliant argument here. If I concede that the Bible is a source of moral guidance, so what? Does that mean I have to be a Christian or even believe in god? Of course not.
Each Sunday after I left the Mormon religion, several men from the church would come to my house to try to win me back. This went on for several weeks. One of them asked me, “Do you believe in Jesus?” I didn’t know what I thought about that. I knew I didn’t believe in miracles or the resurrection, for sure, but the historicity of Jesus was something that wasn’t important to me. He continued, “Who will be your example?” My reply was terse, “Jesus.” I explained that I could use Jesus as an example of how to live, even if he were a fictional character. The point was entirely missed.