Einstein is a contemporary great master of physics; his Relativity is one of the most important theories in the 20th century.
He liked to watch the performance of Charles Choplin, the great humorist. They admired each other and corresponded.
Once, Eintein wrote Choplin a letter that said, 'I loved to watch your performance very much. Almost everybody understands your plays. I believe you will certainly become a great man in the future.'
On reading Einstein's letter, Choplin replied, "I have heard about you for a long time. Nearly nobody understands Relativity. You have already been a great man."
Being easy to understand or hard to understand can equally becomes a standard for being great.