The idea of "Oneness of Bing" is considered the most important and the most focal ontological claim of mystics and theosophists. Some mystic exegetes have been serious advocates of this claim, and hence have introduced it in their esoteric interpretations of the Qur'an. The early mystical Quranic exegeses, however, simply dealt mainly with the internal meaning of the Quranic verses. The main idea of mystics was to extinguish the low desires of the soul and selfhood, leading their soul to divine love and finally to annihilation in God. So there is no hint at the idea of Oneness of Being in their exegeses. Then, instead of the immediate internal and didactic meanings based on intuition and spiritual feelings, they ventured to develop some philosophical and mystical concepts. And finally the ideas of divine love and annihilation in God were led to the idea of Oneness of Being, and in the 7th century the idea was brought into Quranic exegesis by Ibn Arabi. After Ibn Arabi, instead of simple and ascetic mystical exegeses, a period of hermeneutic reflections based on the idea of Oneness of Being began, and this idea was propagated by Ibn Arabi's followers. Sultan 'Ali Shah Gonabadi, the author of Bayan al-Sa'adah fi Maqamat al-'Ibadah, is among the strong supports of this idea, speaking about it in various places. He described this idea by relying on the words of Ibn Arabi and Mulla Sadra, and went to the extreme of excess in interpreting the Quranic verses according to this idea. In addition to surveying the historical background of this idea, its semantic dimensions, and the way it entered into the mystical exegeses, the author has shown the sway of this idea over Tafsire Bayan al- Sa'adah and its comparative interpretation of the Qur'an in terms of this idea, and has presented a close examination of it.