The Quran

۞ اللَّهُ نورُ السَّماواتِ وَالأَرضِ ۚ مَثَلُ نورِهِ كَمِشكاةٍ فيها مِصباحٌ ۖ المِصباحُ في زُجاجَةٍ ۖ الزُّجاجَةُ كَأَنَّها كَوكَبٌ دُرِّيٌّ يوقَدُ مِن شَجَرَةٍ مُبارَكَةٍ زَيتونَةٍ لا شَرقِيَّةٍ وَلا غَربِيَّةٍ يَكادُ زَيتُها يُضيءُ وَلَو لَم تَمسَسهُ نارٌ ۚ نورٌ عَلىٰ نورٍ ۗ يَهدِي اللَّهُ لِنورِهِ مَن يَشاءُ ۚ وَيَضرِبُ اللَّهُ الأَمثالَ لِلنّاسِ ۗ وَاللَّهُ بِكُلِّ شَيءٍ عَليمٌ

Muhammad Asad

God is the Light of the heavens and the earth. The parable of His light is, as it were,¹ that of a niche containing a lamp; the lamp is [enclosed] in glass, the glass [shining] like a radiant star:² [a lamp] lit from a blessed tree - an olive-tree that is neither of the east nor of the west³ - the oil whereof [is so bright that it] would well-nigh give light [of itself] even though fire had not touched it: light upon light!⁴ God guides unto His light him that wills [to be guided];⁵ and [to this end] God propounds parables unto men, since God [alone] has full knowledge of all things.⁶

The particle ka ("as if" or "as it were") prefixed to a noun is called kaf at-tashbih ("the letter kaf pointing to a resemblance [of one thing to another]" or "indicating a metaphor"). In the above context it alludes to the impossibihty of defining God even by means of a metaphor or a parable - for, since "there is nothing like unto Him" (42:11), there is also "nothing that could be compared with Him" (112:4). Hence, the parable of "the light of God" is not meant to express His reality - which is inconceivable to any created being and, therefore, inexpressible in any human language but only to allude to the illumination which He, who is the Ultimate Truth, bestows upon the mind and the feelings of all who are willing to be guided. Tabari, Baghawi and Ibn Kathir quote Ibn 'Abbas and Ibn Mas'ud as saying in this context: "It is the parable of His light in the heart of a believer."
The "lamp" is the revelation which God grants to His prophets and which is reflected in the believer's heart - the "niche" of the above parable (Ubayy ibn Ka'b, as quoted by Tabari) - after being received and consciously grasped by his reason ("the glass [shining brightly] like a radiant star"): for it is through reason alone that true faith can find its way into the heart of man.
It would seem that this is an allusion to the organic continuity of all divine revelation which, starting like a tree from one "root" or proposition - the statement of God's existence and uniqueness - grows steadily throughout man's spiritual history, branching out into a splendid variety of religious experience, thus endlessly widening the range of man's perception of the truth. The association of this concept with the olive-tree apparently arises from the fact that this particular kind of tree is characteristic of the lands in which most of the prophetic precursors of the Qur'anic message lived, namely, the lands to the east of the mediterranean: but since all true revelation flows from the Infinite Being, it is "neither of the east nor of the west" - and especially so the revelation of the Qur'an, which, being addressed to all mankind, is universal in its goal as well.
The essence of the Qur'anic message is described elsewhere as "clear [in itself] and clearly showing the truth" (cf. note 2 on 12:1); and it is, I believe, this aspect of the Qur'an that the above sentence alludes to. Its message gives light because it proceeds from God; but it "would well-nigh give light [of itself] even though fire had not touched it": i.e., even though one may be unaware that it has been "touched by the fire" of divine revelation, its inner consistency, truth and wisdom ought to be self-evident to anyone who approaches it in the light of his reason and without prejudice.
Although most of the commentators read the above phrase in the sense of "God guides unto His light whomever He wills", Zamakhshari gives it the sense adopted in my rendering (both being syntactically permissible).
I.e., because of their complexity, certain truths can be conveyed to man only by means of parables or allegories: see notes S and 8 on 3:7.