Frithjof Schuon said this quote

When people want to be rid of Heaven it is logical to start by creating an atmosphere in which spiritual things appear out of place; in order to be able to declare successfully that God is unreal they have to construct around man a false reality, a reality that is inevitably inhuman because only the inhuman can exclude God. What is involved is a falsification of the imagination and so its destruction.

~ Frithjof Schuon

In relation to Monotheism considered as such, Judaism stabilized but “confiscated” the Message; Christianity universalized but “altered” it; Islam in turn restored it by stabilizing and universalizing it.

~ Frithjof Schuon

As for the negation of the Christian Trinity in the Quran - and this negation is extrinsic and conditional - we must take account of certain shades of meaning. The Trinity can be envisaged according to a vertical perspective or according to either of two horizontal perspectives, one of them being supreme and the other not. The vertical perspective- Beyond-Being, Being and Existence - envisages the hypostases as descending from Unity or from the Absolute - or from the Essence it could be said - which means that it envisages the degrees of Reality. The supreme horizontal perspective corresponds to the Vedantic triad Sat (supraontological Reality), Chit (Absolute Consciousness) and Ananda (Infinite Beatitude), which means that it envisages the Trinity inasmuch as It is hidden in Unity(1). The non-supreme horizontal perspective on the contrary situates Unity as an essence hidden within the Trinity, which is then ontological and represents the three fundamental aspects or modes of Pure Being, whence the triad : Being, Wisdom, Will (Father, Son, Spirit).Now the concept of a Trinity seen as a deployment (tajalli) of Unity or of the Absolute is in no way opposed to the unitary doctrine of Islam ; what is opposed to it is solely the attribution of absoluteness to the Trinity alone, or even to the ontological Trinity alone, as it is envisaged exoterically. This last point of view does not, strictly speaking, attain to the Absolute and this is as much as to say that it attributes an absolute character to what is relative and is ignorant of Maya and the degrees of reality or of illusion ; it does not conceive of the metaphysical - but not pantheistic - identity between manifestation and the Principle; still less, therefore, does it conceive of the consequence this identity implies from the point of view of the intellect and the knowledge which delivers.(1) The Absolute is not the Absolute inasmuch as it contains aspects, but inasmuch as It transcends them; inasmuch as It is Trinity It is therefore not Absolute.

~ Frithjof Schuon

It is impossible to describe a landscape so validly as to exclude all other descriptions, for no one can see the landscape in all its aspects at the same time, and no single view can prevent the existence and validity of other equally possible views.

~ Frithjof Schuon