While you’re thinking …
Einstein published his definitive paper on general relativity (more correctly his general theory of relativity) in 1915.
While it’s possible to understand Einstein’s earlier special theory of relativity with high school mathematics, even the great man himself had to get help with the maths that would be required for general relativity.
The person who made the remark was, at the time, one of the most famous physicists in the world.
Arthur Eddington
There are a number of variants on the Eddington story. In the simple form used in the question, the questioner was said to be a newspaper reporter, while in a more sophisticated version, another physicist, Ludwik Silberstein, was said to have commented to Eddington that Eddington was one of the three who understood the theory. Silberstein, who clearly had himself and Einstein in mind for the other two, mistook Eddington’s delay in replying for modesty and told Eddington not be so shy, at which point Eddington is said to have replied cuttingly, ‘Oh, no. I was wondering who the third might be.’
There was no doubt that Eddington did understand the theory (though in reality there were plenty of others who did, including German mathematician David Hilbert, who, seeing an early version of Einstein’s ideas, nearly beat Einstein to the publication of a final version). Eddington also made considerable contributions to cosmology. But part of the reason he was so well known was because he was the Brian Cox of his day – he was an excellent science populariser.
It was also Eddington who helped cement Einstein’s victory by arranging an expedition to measure the light-bending effect predicted by general relativity during the solar eclipse of 1919. It has since been suggested that Eddington made sure the results supported Einstein’s theory, as his equipment and the conditions made it very difficult to come up with a definitive answer, but as far as the world was concerned, Eddington’s observations turned Einstein from a reasonably well-known theorist into a scientific superstar.
Further reading: Reality’s Frame