The world of consciousness is boundless, which is to say that there are not a finite amount of possible mental states to fit all circumstances. Each of these states, if we were to commit the error of isolating them into supposed distinct realities, is in itself real - such states being for example terror, wonder, sorrow, joy. The works of Homer would seem to operate within an heroic but ultimately rather crude universe of finite states covering the multitudes of experience while we could say that Shakespeare represents the boundlessness of consciousness.
The human mind can conceive of imaginary physical realities, but this cannot be said of psychological/emotional states which if capable of conception must be real, as the mind cannot create beyond itself in these terms. That they can be experienced means they are real. If we allow ourselves a self as a reality, then this self moves within an infinite world of itself, or Self.
To now move to the world of art & in particular art of a religious nature...the essence of great art of this nature such as Bach's Matthew's Passion is not a religious world that it points towards; the music is itself religious. This music is an emanation of a state of consciousness which is itself religious; it cannot simply be said to be about an idea that is religious, but which may not exist.
Art of this nature fails when there is a discrepancy between what the art is 'about', & what it is in itself. This kind of art aspires consciously to being 'of' the elevated heights, but is in itself not intrinsically this reality & so is rightly described as pompous or pretentious. In other words, the art is not the natural outpouring of the artist's inner being but a forced effort to be something he is not. Great art in general & great religious art in particular are of themeselves the world they point towards. This shown especially in some of the images at this site on Russian icons here; an image I would especially mention being The Burial of Christ on the second page. The psychological reality of these images is itself spiritual, & since an experience of the mind is itself real, then these images are their own proof of the spiritual aspect of reality. To stress, they are not 'about' a projected spiritual dimension; this spiritual reality is their very existence as emanations of consciousness.