I believe that the old Gods such as Odin once existed, because there are a lot of records in history of Germany, England and Old Norse texts where people associate Odin with certain powers such as healing, poetry, frenzy, knowledge and more. Odin also was a subject to numerous studies in Germany.
Anyone who believes in mythology, magic, spirituality and the like are entitled to their own opinion. There's no scientific evidence that Gods like Odin actually existed but if anyone practices religion and firmly believes in their faith that they did exist, then who is anyone to say that these people are worshiping fictional figures?
I believe that there is only one God and has only been one God for the entirety of the universe. However, before humans developed a further understanding into how the world works, it makes sense that they would attribute unexplainable actions to other gods, just like the Greeks and Romans did.
The old gods in mythology and literature are myths. They did not exist, but do serve a purpose. The stories about the gods were and are teaching tools for human kind. We all want to know the meaning of life, and the story of the gods help explain it. It helps on the journey of the warrier, which is the journey we all want to take.