This is the wrong question. Einstein was sure that nothing would ever travel faster than the speed of light, but he was wrong. At a certain level, anyway since at great distances galaxies are indeed moving away faster than light.
The question is more accurately formed as “How sure ARE you?”
All knowledge comes in degrees of certainty. We know the earth is mostly spherical with essentially 100% certainty. We know what the speed of light is in a vaccuum within tiny tolerances. We know that certain bacteria can cause disease with near certainty. And so on.