"What do Golden Dawn Magicians ...
... take for granted or assume?"
Golden Dawn Magicians take for granted the existence of the Divine essence of the natural human being, which can be developed, improved, illuminated, and rendered perfect through the practices of the order, which are of the highest possible elevation. They assume that it is possible, through putting into practice certain precise rituals and techniques, the true nature of the human being may be awakened and revealed in its full divine, and not merely material, powers, and that thereby a new world will be revealed.
They assume that there is a much larger world than that which we see as material which is merely only a small part, and that the real world extends into subtle planes invisible to human senses, but perceptible through developing other faculties.
Hence the importance of the development of the Second Body or "Subtle Body," without which perception of the inner worlds remains impossible.