Quoted from EIT37–43 
The doctrine of Unity, that is to say the affirmation that the Principle of all existence is essentially One, is a fundamental point common to all Orthodox traditions , and we can even say that it is on this point that their fundamental identity appears most clearly,  being reflected even in the expression itself. Indeed, when it is a question of Unity, all diversity is erased, and it is only when one descends towards multiplicity that the differences of forms appear, the modes of expression then being multiple themselves (same as that to which they refer) and liable to vary indefinitely to adapt to the circumstances of time and place.
But “the doctrine of Unity is unique” (according to the Arabic formula: Et-Tawhīdu wāhidun), that is to say that it is everywhere and always the same, invariable as the Principle, independent of multiplicity and change that can only affect contingent applications.
So we can say that, contrary to popular opinion, there has never been anywhere any truly “polytheistic” doctrine, that is to say one that admits an absolute and irreducible plurality of principles. This “pluralism” is only possible as a deviation resulting from the ignorance and incomprehension of the masses, from their tendency to attach themselves exclusively to the multiplicity of the manifest:  hence “idolatry” in all its forms, that arises from the confusion of the symbol itself with what it is intended to express, and the personification of the divine attributes considered as so many independent beings, which is the only possible origin of a de facto “polytheism”.
This tendency, moreover, becomes more accentuated as one advances in the development of a cycle of manifestation , because this development itself is a descent into multiplicity, and because of the spiritual obscuration which inevitable accompaniment. This is why the most recent traditional forms are those which must express in the most visible way the affirmation of Unity; and, in fact, this affirmation is expressed nowhere so explicitly and with so much insistence as in Islam , where it even seems, so to speak, to absorb in itself every other affirmation.
The only difference between the traditional doctrines, in this respect, is that which we have just indicated: the affirmation of Unity is everywhere, but, originally, it did not even need to be formulated expressly in order to appear as the most obvious of all truths, because men were then (still) too close to the Principle to misunderstand it or lose sight of it.
Now, on the contrary, it can be said that most of them, fully engaged in multiplicity, and having lost the intuitive knowledge of truths of a higher order, come only with difficulty to the comprehension of Unity and this is why it gradually becomes necessary, during the course of the history of terrestrial humanity, to formulate this affirmation of Unity over and over again and more and more clearly, we could say more and more energetically.
If we consider the current state of things, we see that this affirmation is somehow more enveloped in certain traditional forms, that it sometimes even constitutes their esoteric side… 
This last case is that of Islam, even exoteric; esotericism here only explains and develops all that is contained in this affirmation and all the consequences that derive from it and, if it does so in terms often identical to those we encounter in other traditions, such as that Vêdānta and Taoism, there is no reason to be surprised, nor to see the effect of borrowings which are historically questionable; it is so simply because the truth is one, and because, in this principal order, as we said at the beginning, the Unity is necessarily translated even into the expression itself.
On the other hand, it should be noted, always considering things in their present state, that the Western peoples and more especially the Nordic peoples, are those who seem to experience the most difficulty in understanding the doctrine of Unity, in at the same time that they are more engaged than all the others in change and multiplicity. The two things obviously go together, and perhaps there is something there that is due, at least in part, to the living conditions of these peoples: a question of temperament, but also a question of climate, one being, moreover, a function of the other, at least up to a certain point.
In the countries of the North, where indeed the solar light is weak and often veiled, all things appear to the eyes with an equal value, so to speak, and in a way which affirms purely and simply their individual existence without letting anything glimpse beyond; thus, in ordinary experience itself, one truly sees only the multiplicity.
It is quite different in countries where the sun, by its intense radiance, absorbs, so to speak, all things into itself, making them disappear before it as multiplicity disappears before Unity, not that it ceases to exist according to its own mode, but because this existence is strictly nothing with regard to the Principle. Thus, Unity becomes in some way perceptible: this solar blaze is the image of the fulguration of Shiva’s eye, which reduces all manifestation to ashes.
The sun imposes itself here as the symbol par excellence of the One Principle (Allahu Ahad), which is the necessary Being, the One who alone is sufficient unto Himself in His absolute plenitude (Allahu As-Samad), and on whom entirely the existence and subsistence of all things, which outside of Him would be nothing.
“Monotheism”, if this word can be used to translate Et-Tawhīd, although it somewhat restricts its meaning by almost inevitably suggesting an exclusively religious point of view, “monotheism”, we say , therefore has an essentially “solar” character. Nowhere is it more “sensitive”/f: sensible than in the desert, where the diversity of things is reduced to a minimum, and where, at the same time, mirages bring out all that is illusory in the manifested world. There, the solar radiation produces things and destroys them in turn; or rather, because it is inexact to say that it destroys them, it transforms them and reabsorbs them after having manifested them.
One could not find a truer image of Unity unfolding outwardly in multiplicity without ceasing to be itself and without being affected by it, then bringing back to itself, always according to appearances, this multiplicity which, in reality, It has never come out of it, because there can be nothing outside the Principle, to which nothing can be added and from which nothing can be subtracted, because It is the indivisible totality of this unique Existence. In the intense light of the countries of the Orient, it is enough to see to understand these things, to immediately grasp their profound truth; and above all it seems impossible not to understand them thus in the desert, where the sun traces the divine Names in letters of fire in the sky.
by René Guénon
Gebel Seyidna Mousa,
23 shawal 1348 H. Mesr,
Seyidna El-Hussein, 10 moharram 1349 H.
(anniversary of the battle of Kerbela). 
Malik and al-Shafiʿi’s Understanding of Tawhid
Progressing toward Tawhid - Unity
Unity - Al-Tawhid
No True Reality but the Reality of the Real
From Anti-Tradition to Counter-Tradition - From The Reign of Quantity and the Signs of the Times, Sh Abd Al Wahid Yahya - René Guénon
Insights Into Islamic Esoterism and Taoism - René Guénon - Google Books; chapter titles by the editor, translated from the French edition. ↩
common to all Orthodox traditions - meaning to their primordial core, before all those deviations and interpolations:
"Of all the historical dispensations of the true Religion, Islam alone holds true until the end of time while all others are abrogated and no longer efficacious as spiritual paths, nor incorrupt as Divine Self-disclosures. Hence the Prophet Muhammad, upon him blessings and peace, alone is to be followed until the end of time while all the others came and went; and he alone is to be universally followed while they came only for their own people including Jesus who came only for the Israelites."
"Hence Judaism, Christianity, Vedantism (a sect of Hinduism), any and all other purported paths of enlightenment and religious laws can no longer help the spiritual wayfarer to reach the supreme good – which is knowledge of God – except insofar as a terminally-stationary plane can help the traveller fly to his destination: he is inside the plane, the plane is thought by those in it to be fuelled and ready, but it no longer takes off and never will again." Sh G F Haddad
There is the example of the Vedas, meaning "knowledge"/ Upanishads, although in this world of ignorance, who would benefit from it?
"The Vedas deal mostly with fruitive activities to gradually elevate the general public from the ﬁeld of sense gratiﬁcation to a position on the transcendental plane."
or: to the multiplicity of manifestation ↩
The author uses the term Islamism instead of Islam in this article. ↩
[text continues:] that it sometimes even constitutes their esoteric side, taking this word in its broadest sense, while that, in others, it appears to all eyes, so much so that we no longer see anything but it, although there are certainly, here too, many other things, but which are no longer only secondary to that one. ↩
March 23, 1930 ↩