Nitpicking: It is not possible to derive the theory of relativity from medieval knowledge. At least not in any way resembling how the development actually happened, that is, heavily influenced by observations (or rather an iteration of speculation and observation). [Unless one counts both Galilean relativity and some parts of the theory of electromagnetism as being contained in medieval knowledge.]
Perhaps it would be possible if the classically observed reality turned out to be possible (consistent under some reasonable conditions) only as a consequences of some deeper, sufficiently determined theory; but that would go far beyond our current level of knowledge, at least as far as I can say.
Perhaps it would be possible if the classically observed reality turned out to be possible (consistent under some reasonable conditions) only as a consequences of some deeper, sufficiently determined theory; but that would go far beyond our current level of knowledge, at least as far as I can say.