He is a mystery because none can know Him by the natural instruments of the senses and mind. And even when we go beyond the mind and develop subtle senses, yet we can at best have only a glimpse of Him here and there. And when with great tapasya and an act of Grace we are led to an identification with Him we cannot express the experience as our thought and speech are very limited and finite and hence unable to reveal the Bliss and Splendour hidden in His infinite consciousness and being.
As to the greatness and the absoluteness, can there be any doubt about it? A look at the material universe and trillions of stars and countless creatures interwoven with such a delicate harmony is evidence enough. And already people talk about millions of universes of different types. The Source, the Creator is always greater than the creation and would exist even if creation vanished in a moment.
Affectionately,
Alok Da