Before the advent of general relativity, Newton's law of universal gravitation had been accepted for more than two hundred years as a valid description of the gravitational force between masses, even though Newton himself did not regard the theory as the final word on the nature of gravity.
However, the classical Newton's theory suffered from two major inconsistencies:
- - it was unable to predict the anomalous rate of precession of the perihelion of Mercury's orbit which was first recognized in 1859 as a problem in celestial mechanics by Urbain Le Verrier.
- - it was incompatible with the Special Relativity's postulate imposing a natural cosmic speed limit - the speed of light, because in Newton's theory, the gravitational force acts instananeously across any distance .
From the last point of view, General Relativity could be seen as an attempt to reconciliate gravity and special relativity. It took Einstein a journey of no less than eight years of hard conceptual and mathematical work before he finally succeeded in resolving these difficulties.
The first step of it will be to postulate The Equivalence Principle, introduced in 1907 in the long review article on Relativity commissionned by the editor of the Jahrbuch der Radioaktivität.
 You can watch this nice video for more information about the Mysterious Orbit of Mercury.
 In his paper The Foundation of the Generalised Theory of Relativity, Einstein paid special tribute to his friend Grossman for his priceless help in these terms: "Finally in this place I thank my friend Grossmann, by whose help I was not only spared the study of the mathematical literature pertinent to this subject, but who also aided me in the researches on the field equations of gravitation. "