Bismillah al-Rahman al-Rahim
“Say: Who is Lord (Rabb) of the heavens and the earth? Say: Allah! Say: Take you then (others) beside Him for protectors (awliya) , which, even for themselves, have neither benefit nor hurt? Say: Is one whose sight is blacked out on an equal footing to one with clear vision, or is darkness equal to light? Or assign they unto Allah partners who created the like of His creation so that the creation (which they made and His creation) seemed alike to them? Say: Allah is the creator of all things, and He is the One, the Almighty.” (Qur'an 13:16)
This verse uses the command “Say” (Qul) multiple times - where God commands the Prophet to repeat to the people the exact words given in this verse. The numerous instances of “Qul” in a single verse indicate that the questions and answers in this verse are very fundamental ones and so great emphasis is laid upon them through the repeated use of “Qul”. The Prophet is directed precisely how to phrase the question and precisely what answer to give through a progression of five Quls. Great weight is given to the phrasing through this methodology indicating we need to pay close attention.
The first two “Quls” are used to ask the question “Who is the Lord (Rabb) of the heavens and the earth?” and to answer it with a single word, Allah.
Elsewhere, the Qur'an (in verse 17:110) says, “Say: Call upon Allah or call upon the All-Merciful; whichever you call upon, to Him belong the most beautiful Names.” (Qur'an 17:110). But here (in Qur'an 13:16), in response to the question ‘who is Lord of all existence’, the answer is only one thing, the name ‘Allah’. There is no elaborate descriptive answer, no mention of descriptive qualities of God such as Rahman and Rahim such as in the Bismillah or in other verses.
One possible reason for this is that the name Allah is considered to be the name that points at God's Essence, that subsumes all other names. He has many names which denote his qualities. Some of these are names that describe his incomparability and transcendence while others describe His relationship to creation - these are names that describe attributes we can understand since we ourselves display them in some manner - names such as hearing, seeing, generous, responder, knower, majestic, powerful. These qualities have been woven into creation so that they are immediately familiar to us even while only God contains them in an all-encompassing absolute manner. They describe the qualities of relationships between God and creation.
While all the Divine Names point in different ways to His Essence, the name Allah is an all-encompassing pointer. As such, it is rare to use this name (Allah) for anything but God, while the other names are used to reference things in creation and can even be used as personal names for ourselves. We can say that someone is generous, that the universe is majestic, that a person is merciful. We can name a person Jamal or Rahman or Adill, all of which are references to qualities by which God names himself. But we cannot use the word Allah in a similar sense. It has only one referent and is not meant to be used for any created thing. It is the primary referent we have for the all-encompassing Essence of God - Allah. All other names are describing qualities and relationships that emerge from the Essence and which are subsumed by the Essence.
So, in verse 13:16, the Prophet is commanded to indicate that the answer to the question (Who is Rabb) is only one thing - an emphatic answer that cuts past all relativities and all words or phrases that might allow more than one meaning to emerge, that may accidentally be taken to refer simultaneously to God and to an aspect of His creation (as with a quality of God such as beauty). All this is instantly sideswiped by pointing directly to God's essence - unfathomable, unknowable and yet named by a word which is used to indicate exactly this unfathomable mystery that is simultaneously at the core of all existence and which is yet beyond and above and encompassing all existence.
In Sura 16:74 (Al-Nahl, The Bee) there is a prohibition on coining similitudes for Allah, although God Himself can coin similitudes as He does in many verses - but we are not to engage in making analogies for His Essence. The verse says: “So coin not similitudes for Allah. Lo Allah knows; you know not.” (Qur'an 16:74)
As Ali ibn Abu Talib states:
“Now a thing is only compared with its like. As for what has no like, how should it be compared with what is other than its like (mithal)? For He (Allah) is greater than that the minds of men should delimit Him through (their limited) thought. So whoso describes Allah (that is, His Essence) - glory be to Him - has given Him a partner (that is, the description of Him).” 
And as the Qur'an says:
“Glory be to your Lord, the Lord of inaccessibility, above what they describe.” (Qur'an 37: 180)
Because of this there is a well developed negative theology in Islam concerning God's Essence - while its positive theology focuses primarily on His attributes. Any notions we have of God's Essence are fated to be weak, fuzzy analogies - like a heavily pixellated image on the cusp of becoming an indiscernible blur or in danger of being blatantly misinterpreted due to our inability to resolve the image.
So when it comes to matters concerning His Essence, God does not leave the matter to us, but takes charge directly of the question and answer process. As a result the whole passage consist of a series of Quls.
God Himself specifies the question that should be asked, and he specifies the answer that should be given. He wants to direct attention to several things - one is the nature of His Lordship (signified by the word Rabb) - the other is His Essence from which everything flows. The reason He does the asking and the answering is because “all those who want to make statements about God (in His essence) are wrong, for they fail to say anything about Him (about His essence) for no quality or description or striking of a likeness can express God.”  We can't make statements about something that is fundamentally ungraspable by our minds. We can only use a term which signifies something which is beyond our farthest understanding, greater than what our minds can attain.
This can be seen in the story of Abraham and his spiritual vision where he sees a star, the moon, and the sun each as a Rabb (as an indicator of God) and eventually rejects striking any kind of likeness for God with anything in creation. In the vision, Abraham then passes on to an acknowledgement of God's incomparability. He perceives three heavenly lights each more intense than the previous one but each one is subject to laws and to time since each one (the star, the moon, and the sun) sets (each is demarcated and limited and subject to natural laws). So then he turns to face what is beyond appearances and beyond all creation and beyond all limitations. He says: “For me, I have set my face, firmly and truly, towards Him Who created the heavens and the earth, and never shall I give partners to God.” (Qur'an 6:79) He says, “never shall I give partners to Him” because he is referring to that Essence which is beyond all comparison and similitudes and since there is no similitude possible, there is no partner possible. There can only be one absolute, unlimited being since if there were two or more then the phrase absolute and unlimited can no longer apply, and since there is only one, there can be no likeness, no similitude for it. As the Qur'an says, “Nothing is like Him.” (Qur'an 42:11)
He is the cause of all things and the creator of all things but not in the sense of an inventor or an artist or a builder whose creation continues to exist without its creator since it is made of materials that are separate from the creator. “But God provides the creature with the whole of what it is, with form as well as matter and substance.”  so its being is sustained entirely by Him and it cannot exist without this continued support. Its very being or existence is generated and sustained entirely by God. It has no ontological independence of any kind at any level. And this is implicit in the use of the word Rabb.
Rabb denotes a very close relationship with the created world of existence. Rabb indicates both ownership and mastery as well as nurture and support for something that is totally dependent. The ownership is not like owning something independent of you but like ownership of something that cannot even continue to exist without you. As weak examples, your hand is an integral part of you and is dependent on you, or an image formed consciously in your mind is dependent on you for its moment by moment existence - you are its Rabb. Allah is Rabb in the sense that everything is poor towards Him, dependent on Him, and “He is the Rich, the Independent” - His Lordship emerges out of His essence and everything is dependent upon and beholden to His Essence.
A question that arises from looking at a verse like 13:16 is how much of this meaning could the idolatrous Arabs of that time have possibly understood. By analogy, we can look at a form of Hinduism such as Advaita Vedanta where Brahma is a unifying divine principle which manifests in numerous forms. In this form of Hinduism, Brahma is the only true reality and everything else is false, an illusion. Though Hinduism contains multiple lesser deities, nevertheless Brahma is the all-encompassing essence from which everything emerges. The same statement made in this Qur'anic verse could apply. The Arab idol worshippers also knew Allah as the overarching Divine Principle that overshadowed all existence - they knew this even while they had lesser gods to whom they turned for help and protection - Allah was not new to them. So this Qur'anic argument in 13:16 is constructed to confront them with what is at the very heart of their own belief system and to use this argument to free them from all the additional false and unnecessary constructs that were added on over centuries. In essence, it is telling them, turn to the One whom you yourself acknowledge as the highest principle since He is the source of everything. 
The Quls in this verse are like repeating peals of thunder. The questions and the responses rumble forth emphatically like storm clouds and flashing lightning that speak with unequivocal prominence.
1 – “Say: Take you then (others) beside Him for protectors (awliya), which, even for themselves, have neither benefit nor hurt?” (Qur'an 13:16)
Existence itself, including the existence of those to whom you turn for protection emerges from Him. Their very being is dependant on Him. They are subject to natural laws, they are subject to decay and disintegration. It is interesting how the prophet is commanded to say that these awliya that people take have neither benefit nor hurt. This shows the degree of dependence of all levels of creation on God.
This verse talks about the negative form of “wala”. The root meaning of awliya and wala is things that are in such close proximity of each other that there is no distance between them. It is used in describing nearness, love, guardianship, control. Here, it refers to a relationship that includes a flow of strong, almost overwhelming, influence, of control. It is this type of relationship that is disallowed in this verse. You cannot enter into a relationship in which those firmly opposed to belief are your awliya - your influencers, your guides, your commanders, your sole protectors. This does not mean that there cannot be friendship and good relations and companionship and exchange of ideas - it does mean that the flow of powerful controlling influence should be prudently hedged against. ↵
2 – Ali (a.s.) on God's transcendence - translated by William Chittick in “A Shi'ite Anthology” ↵
3 – Meister Eckhart, page xv, “Meister Eckhart, from whom God hid nothing”, Leominster, Herefordshire: Gracewing, 1995 ↵
4 – Meister Eckhart, “Meister Eckhart, from whom God hid nothing”, Leominster, Herefordshire: Gracewing, 1995 ↵
5 – Recently, some small factions of Muslims have stirred up controversy by arguing that Christians and other non-Muslims should be prohibited from using the name Allah as their reference for God when translating their religious writings to Arabic or when speaking in Arabic because of the exclusive uniqueness of the name Allah. Besides the fact that this is completely unenforceable, there is little or no merit to the argument. The pre-islamic arabs used the name Allah and no prohibition was laid on them - rather the meaning was simply re-clarified in the Qur'an. Christians do not believe in a different God - whatever layers of elaborate theologies have arisen over time, it is nevertheless true that there remains an essential divine principle with an unknowable essence. In Arabic, this divinity would be Allah. Every traditional religion has a divine principle. Simply because additional layers of theology have been added on to the religion does not negate the possibility of referring to this highest divine principle - Allah is the name for this principle in Arabic. If we attempt to needlessly legally enforce subtle distinctions, this would also eventually recoil on the Muslims and add yet another battle to the many existing conflicts between differing Muslim mahdhabs and sects with their own theological variances and approaches. ↵