This may be somewhat silly to ask, but I couldn't resist the temptation. The idiosyncratic physicist Richard Feynman was once asked
If, in some cataclysm, all of scientific knowledge were to be destroyed, and only one sentence passed on to the next generation of creatures, what statement would contain the most information in the fewest words?
And his answer was that the world is made of atoms and the qualitative description of attractive electromagnetic interaction at long distances and repulsive nuclear interaction at short distances that governs the atoms:
I believe it is the atomic hypothesis that all things are made of atoms — little particles that move around in perpetual motion, attracting each other when they are a little distance apart, but repelling upon being squeezed into one another. In that one sentence, you will see, there is an enormous amount of information about the world, if just a little imagination and thinking are applied.
If any mathematician was to be asked such as question, but restricted to mathematics, and perhaps, for it to be more interesting and more answerable, not necessarily restricted to a single sentence, but perhaps several ones, or several key concepts or ideas, what would those be?
And to make it possible for your answer to be appreciated and be interpreted not solely as a personal preference, provide an explanatory argument especially from math history point of view like @Anupam did, or a logical one. Feynman's answer is more or less easy to appreciate, as without knowing about atoms, people will be limited to many phenomenological interpretations of the nature, and not start to look at inner-workings of the world from that level of microscopic angle.