The central doctrine concerning the ultimate nature of reality has usually been called wahdat al-wujûd or the (transcendent) unity of Being. This cardinal doctrine, which is not pantheism, or pan-entheism nor natural mysticism as Western orientalists have called it, is the direct consequence of the Shahâdah. It asserts that there cannot be two completely independent orders of reality or being which would be sheer polytheism or shirk. Therefore, to the extent that anything has being it cannot be other than the Absolute Being.
The Shahâdah in fact begins with the lâ, or negation, in order to absolve Reality of all otherness and multiplicity. The relation between God and the order of existence is not just a logical one in which if one thing is equal to another the other is equal to the first. Through that mystery that lies in the heart of creation itself, everything is, in essence, identified with God while God infinitely transcends everything. To understand this doctrine intellectually is to possess contemplative [intuitive] intelligence; to realize it fully is to be a saint who alone sees 'God everywhere'.
From: Ideals and Realities of Islam, S H Nasr