https://www.quora.com/Are-mass-and-energy-equal-and-exchangeable
We could (indeed, theoretical physicists routinely do) measure time and space using the same unit (e.g., seconds vs. "light seconds").
Speed, as you know, is length divided by time. But if length is really the same as time, then length divided by time is really nothing... speed has no units of measure in this scheme.
That takes us to energy, which has units of mass times speed squared. (The Newtonian kinetic energy, for instances, is one half mass times speed squared.) So if speed has no units, then mass times speed squared is just mass again.
So then, so long as we accept that time and space really are the same thing (different directions in the same spacetime) then mass and energy are the same thing, too.
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