Einstein changed the assumptions of absolute space and time with his 2 postulates of relativity.
Postulate 1: Laws of physics are same in all IRFs.
This means that there is no preferred frame of reference, no absolute or unique position in the universe, and no experiment that can show that an observer is at rest or moving at a constant velocity.
Thus, all uniform motion is relative.
Postulate 2: Speed of light in a vacuum is constant/invariant, denoted c, for all IRFs
This is an extension of Maxwell’s Theory of Electromagnetism, where Einstein states that the generic velocity v = \frac{d}{t} doesn’t really work for light because of this postulate.
Though today we see Einstein’s work as a total revolution in relativity and nuclear physics, Einstein himself thinks his postulates and theories are just a completion of Newtonian physics, not a revolution.