Apparently, Dawkins is alluding to Fermat's Last Theorem. Several issues:
i) It's prudent not to endeavor to be more clever than you are, because a failed attempt will make you look less clever than you aspire to be.
ii) I'm no expert, but it's my impression that Dawkins bungled the formulation by omitting n>2. He should leave math to mathematicians and stick to evolutionary biology.
iii) What is the implicit argument in his tweet? I suppose it's something like this: Gospel writers can fabricate reported miracles, but a scientific or mathematic theory, theorem, conjecture, or discovery that's centuries ahead of its time can't be faked. It would be unmistakably anachronistic and undeniably impressive. Assuming that's in the ballpark of what he was gesturing at:
iv) Since modern mathematical and scientific notation didn't exist in the 1C, how could that be expressed in Aramaic or Koine Greek?
v) Since the formula would be unintelligible to Christian scribes (as well as readers), it would almost inevitably be miscopied.
vi) Unbelievers don't think the Gospels reliably record the sayings of the historical Jesus. So even if the Gospels contained something like Fermat's Last Theorem, unbelievers could chalk that up to the narrator, or his hypothetical source, rather than Jesus. He just put that in the mouth of Jesus.
vii) Assuming (ex hypothesi) that Jesus said that, it might prove that he was a mathematical genius, but human genius is no proof of deity, and it's irrelevant to his mission as the Redeemer and eschatological judge.
viii) Is Fermat's Last Theorem especially impressive–or the solution!
ix) Premature scientific and mathematical theories and discoveries would alter the future course of history by kickstarting math and science. It's like those scifi scenarios in which the time-traveler inadvertently changes the future because he carries his modern knowledge with him when he goes back in time, where he says or does something that seeds the past with future know-how or technology. Indeed, he becomes trapped in the past because the future from which he came no longer exists, so he's now unable to return to that point in the erstwhile timeline.