The whole E=mc^2 equation was the proof used to back the theory that energy and mass in motion were the same thing.
Regarding Enthalpy and free energy, those where the main 2 reasons why this theory may have been created. It solves the reason why we could lose mass through emissions of radiation in a reaction - they had to relate energy to mass.
Nothing could explain things like radiant energy (example, in the form of gamma, or visible light) and certainly nothing could hope to explain heat.
You can't have a quantity of heat and put it somewhere and you can't have a bucket full of light itself. So this equation was an attempt to explain that mass itself is in constant motion, often erratic, and mass is therefore 'Energy'. The general belief is sub atomic particles are made of energy ... electrons aren't made of electron material, they consist of packets of energy because at that size they are no longer capable of emitting light or contributing light energy, they cannot be physically seen. The millions of definitions of E=mc^2 will surely explain this stuff anyway.
Jesus, had very little to do with any of this - Though Jesus was most likely modeled from our prime example of the E=mc^2 equation .... our Sun.
E=mc^2 is only 1 explanation, sure there could be other explanations but this one more or less relies on the relationship of mass to energy, to light .... if those aren't related.... E=mc^2 will be useless and God will still have nothing to do with any of it.