To understand the general theory of relativity, we need to learn differential geometry, and it is not the kind of differential geometry for undergraduates in university mathematics department, and we need to reach the level of differential geometry for postgraduates in university mathematics department. When the general theory of relativity first came out, almost no one in the world understood differential geometry, so it was said that only five people in the world could understand it at that time.

Of course, things are different now. The basic knowledge of physics and mathematics is differential geometry, and general relativity is no longer difficult.