So we know Einstein had a great friend and helper Marcel Grossmann.

Read more: http://www-history.mcs.st-andrews.ac.uk/Biographies/Grossmann.html

In August 1912, Einstein arrived back at the Eidgenössische Technische Hochschule in Zürich having been appointed to the chair of theoretical physics. He was struggling to extend his special theory of relativity to include gravitation and immediately began collaborating with his old friend Grossmann. It was Grossmann who pointed out to him the relevance to general relativity of the tensor calculus which had been proposed by Elwin Bruno Christoffel in 1864, and developed by Gregorio Ricci-Curbastro and Tullio Levi-Civita around 1901. Einstein, who previously had played down the importance of mathematics, was quickly convinced by Grossmann’s expert explanations. Einstein wrote to Arnold Sommerfeld on 29 October 1912:-

I am now working exclusively on the gravitation problem and believe that I can overcome all difficulties with the help of a mathematician friend of mine here[Marcel Grossmann]. But one thing is certain: never before in my life have I toiled any where near as much, and I have gained enormous respect for mathematics, whose more subtle parts I considered until now, in my ignorance, as pure luxury. Compared with this problem, the original theory of relativity is child’s play.

Special honour is Given to Marcel:

Finally we should mention the honour given to Marcel Grossmann by naming the series of conferences, the Marcel Grossmann Meetings (on Recent Developments in Theoretical and Experimental General Relativity, Gravitation, and Relativistic Field Theories). These conferences, begun in 1975, are international meetings held every three years, which provide opportunities for discussing recent advances in gravitation, general relativity and relativistic field theories. They emphasise mathematical foundations, physical predictions and experimental tests.

Here he acknowledged Grossman in his notebook:

http://www.pitt.edu/~jdnorton/teaching/GR&Grav_2007/Einstein’s_Zurich_Notebook.html