User blog comment:Miskos3/Fairy Tail 535: The Greatest Power, Review/@comment-29778526-20170513015909/@comment-7418318-20170513065219

Leaving aside the two obvious possibilities that Mard Geer was lying or didn't open the book out of his religion-like reverence to Zeref and E.N.D. there is also a third possibility:

There were around 400 years in which Natsu basically didn't exist. I mean the time between his entering the portal in the distant past and exiting it in X777. From the perspective of the book (lol...) in that time there was no Natsu and maybe it was impossible to open the book while there was no body around.

That might have led Mard Geer to conclude that E.N.D. was sleeping in his book-form and needed to be revived. And when the usual method of rousing a Demon failed, he came up with that elaborate bullshit plan of his involving the FACEs.