I'd really never studied tensors until I started studying the Einstein Field Equations. Since then, I have realized they are fairly common tool in physics and pretty basic to understanding many areas. But for the majority of my initial introduction to special relativity, I never needed tensors. They never really came up. As I try to think about how Einstein moved from creating special relativity to general relativity, I wonder how he knew that tensors would be useful for general relativity.
What led him to think tensors would be needed to formulate the EFEs?