Legibility, in fact, becomes of minor importance, since calligraphy always conveys and constitutes by its very essence the central symbol of Faith. A closeness to Plato's view that writing is the geometry of the soul is evident. Monumental architectural inscriptions, like those in tiny household objects, were more often observed and admired than read. If religious in content, that is, if extracted from the Quran, for most Muslims the recognition and the understanding of part of the inscription sufficed for him or her to know what the rest of the inscription said and for the viewer to recognize that he found himself before a building or an object emanating from his own culture and tying him to his religious brethren. Such inscriptions, however, if unread or even illegible to the mass of believers, served a symbolic function confirming the power and rectitude of Islam simply by their presence.
Islamic calligraphy takes on iconographic function.
I suppose an analogy would be that it is probable that few Western believers understood the complex theology behind the complex sculptures on gothic churches, just as few Eastern believers comprehended the abstruse iconography of the temples. The fact of intricate thought given visible form may particularly please intellectuals and theologians, but such an intellectual satisfaction is not perhaps the substance and essence of religious faith to the mass of people. The visible testimony of Islam on buildings, objects and elsewhere, was an affirmation of religious and cultural belonging and it was this affirmation which held a vital social function. The role of calligraphy in uniting believers in Islam and in strengthening their feeling of having their own religious identity cannot be overstated. Civilization and sedentary culture developed rapidly throughout the expanding Muslim empire in the early years.