Bismillah al-Rahman al-Rahim
“We send not down the Angels save with the Truth (haqqi), and in that case no respite would they have. We have sent down the Reminder (zikru), and We assuredly are its Guard (haafizun).” (Qur'an 15:8) 
“We” indicates plurality. When God refers to himself in the plural, despite His Oneness, it possibly signifies His Names and His qualities that pervade all existence. The names signify a single Reality that assumes many qualities as it manifests itself and that underlies all existing things. When the Qur’an uses “I”, it speaks from God’s absolute (and ultimately unknowable) unity. When it uses “We”, it speaks from God’s turning towards His creation through the medium of His names which denote the qualities and relationships that manifest throughout creation. It is through this that it becomes possible to see endless signs and pointers to God within the diversity of the phenomena in the universe. Wherever creation is, God accompanies it in respect of His many names which infuse all of creation but which nevertheless signify a single Reality. “We” indicates that God underlies every aspect of creation, even in its infinite multiplicity.
Revelation is referred to as zikru, as a reminder. Why is it a reminder, and what is it supposed to remind us of? What are we supposed to be remembering?
In part we are remembering the Divine image within us, the Divine spirit which lies at the core of our existence, we are remembering that we were created in a high station and then descended to the station of the material world. Remembering God through remembering this is one of the means through which we begin to unfold the potentials of human existence. The Qur’an says “We created man of a high stature, then we reduced him to the lowest of the low.” (Qur’an 95:9) The higher nature never vanished or disappeared - it became shrouded and concealed by other lower aspects - the sending down was a veil, a concealment of the potential stature residing within us, just as the multiplicity of creation can be a veil over the unity that underlies it. Our potential is in our ability to awaken, even if only to a small extent, that nature which is now cloaked and concealed by a lower, mundane nature. That lower nature causes ghafalah (distraction or forgetfulness) of what we really are. It is like falling into a drowsy dream and revelation is there to wake us to a truer nature and a deeper reality. Through this awakening, the lower, mundane nature also assumes the luminosity of the higher nature concealed within us.
Revelation is a reminder of this purpose and is a means to beginning the journey to recover the inner luminosity that awaits within us like a seed that is waiting to sprout and grow and that is watered and strengthened by “the reminder”, by the revelation.
Although zikr is the word used for reminder, in this context it is generally considered to refer to revelation in general, and the Qur’an specifically. The phrasing (nazzalna) indicates that it was sent down in stages or segments. This can refer to the fact that the Qur’an was revealed gradually over a period of 23 years.
It can also refer to the stages of descent from a higher reality to a lower reality until the final descent culminating in the encoding of the Qur’an into the words and phrases and rhythms of the book we hold in our hands. The descent of the Qur’an is not like the descent of water from a cloud to the earth in the sense that when the water leaves the cloud the cloud is diminished by that amount - the drop moves down to a place that the cloud is not. The descent of the Qur’an from the Divine treasuries to the umm al-kitab (mother of the book), then to the heart or innermost consciousness of the Prophet, and then to the Arabic words that make up the physical book, refers to a descent in which the source is not in any way diminished or attenuated by the process of descent. Rather the thing that descends has a continuous connection with its place of origin, it is a manifestation of its source but a manifestation that has undergone a process of descent so that it is accessible to us in our human circumstances. It undergoes a descent to a lesser intensity of being. This descent is necessary, otherwise a direct manifestation from a higher Reality would overwhelm the lower reality. When Moses asked to see a manifestation of God, the mountain turned to powder when the veil was briefly lifted (Qur’an 7:143). He (Moses) said, “My Lord, show me Yourself that I may look at You.” The reply is: “You cannot (bear to) see Me, but look upon the mountain; if it is capable of retaining its form, then you will be able to see Me. But when his Lord manifested to the mountain, He rendered it into utterly pulverized, crushed powder, and Moses fell as if dead.” (Qur’an 7:143) Likewise, if the source of revelation in its true ontological reality was manifested to anything rooted in material existence,“you would see it (existence) rent asunder.” (Qur’an 59:21) So a process of descent is necessary since material reality is utterly transformed or demolished by direct exposure to a higher more intense metaphysical reality, a more intense level of being.
This is why Angels are not sent down in their true form and ontological reality, since the intensity of their substance, of the higher reality they dwell in, would overwhelm this material reality. When, in the Qur’an, Angels appear to Prophet Ibrahim and to prophet Lut, they clothe or project themselves in a human form. When Gabriel manifests to the Prophet in a somewhat higher form, the Prophet feels as if he is being crushed by Gabriel's presence. If they were to appear in their true reality, such a manifestation would rend apart our reality. Our material forms would not be able to hold together under the intense presence of a higher order of existence, of being. This illusory plane would vanish away in the presence of a much deeper reality, a much truer reality.
By the process of descent, a de-intensification takes place and by descending to the level of words, the Qur’an clothes Divine manifestation in garments of powerful and beautiful language - but also stretches the limits of a human language that is shattered trying to bear the weight of Divine meanings. The Qur’an’s language in turn speaks to receptive hearts and it is there in the encounter between the heart and the Qur’an that the Divine manifestation can occur. As Ali says in the Nahj-al-balagha, “He (God) manifested Himself to them through His book.” (Khutba 147)
The phrasing “haafizun” indicates that the reminder is continuously guarded so that it could be said to be protected without even the minutest interruption in that protection, just as a haafiz commits something so completely to memory that what is in their memory is guarded from even the slightest change. Likewise, the reminder(the Qur’an) is protected in its essence from alterations, additions, or deletions of the original.
The guarding of it could also refer to the sense that its deepest essence is guarded from being comprehended, so that “none shall touch it but the purified”. (Qur’an 56:79) That is, its inner content is concealed or clothed in an outer wrapping such that its spiritual realities and subtleties are accessible only to those whose hearts are purified from the illusions and distractions of the world. As Rumi says, “cleanse yourself, for His dwelling place is your innermost thoughts.” This meaning is also indicated by Jafar al-Sadiq who says that the guarding means that revelation acts as a protection for the one who actively treats it as a remembrance and begins to awakens their deeper nature. It acts as the opposite, as a bringer of adversity, to those who cast aside this inner aspect of revelation and approach it as a document to be understood primarily in a literal sense, a tool for manipulating societies and regimenting people rather than as a profound remembrance that transforms our inner nature and so also our societies.
Because of the open phrasing of these verses, “reminder” also can have a wider meaning than only referring to the Qur’an. It could also refer to previous revelations and the religions that arose from those revelations, although the guarding may have a different meaning since those religions took shape largely through oral traditions and what was written down came much later. However, each religion originated as a reminder and to the extent that it fulfills the function of making remembrance possible for those who earnestly seek to awaken their true humanity and to seek God, it has been guarded.
In a sense, all of creation is revelation, the specific religious revelations are to awaken us to our place within an unfathomably vast and profoundly deep creation. The reminder is then also an indicator that our innermost being is also a revelation, a guarded revelation that retains its original purity even as we stray and wander and turn away from it in our worldly lives. “What they do has become like obscuring rust upon their hearts.” (Qur’an 83:14) This inmost, God-given aspect of our being, may become covered over and obscured by our immersion in the distractions of life, but under the obscuring tarnish the inner kernel remains. “There is a polish for everything that takes away rust; and the polish for the heart is the remembrance of Allah.”