The mysterious what, exactly? AG, I assume that you believe in good and evil, or maybe you do not. If you do, how can mysterious evil be the source of anything true, other than itself?
The most beautiful and deepest experience a man can have is the sense of the mysterious. It is the underlying principle of religion as well as all serious endeavour in art and science. He who never had this experience seems to me, if not dead, then at least blind. To sense that behind anything that can be experienced there is a something that our mind cannot grasp and whose beauty and sublimity reaches us only indirectly and as a feeble reflection, this is religiousness. In this sense I am religious. To me it suffices to wonder at these secrets and to attempt humbly to grasp with my mind a mere image of the lofty structure of all that there is.
It seems to me (since He alone is the uncaused cause) - that He would have a method to reveal Himself to the spiritual creatures who would become members of His family.
Since one cannot know what good (a property of God) is without seeing it in contrast to what it is not (evil) I deduce that God would show both to His beloved creatures in order to reveal Himself. The same would apply to truth v lie, life v. death, courage v. fear, love v. hate, health v sickness and so on.
My two cents, FWIW.