NB Einstein actually ponder over to call his theory the theory of the absoute because besides showing that space, time simultaniety, mass (and more) were relative he also showed what was absoulte

e.g.  c  the velocity of light in vaccum and "distance" in spactime

I don't know the sayings in english but in Sweden many people says "Allt är relativt"  All is relative.

But this is not what Einstein said.