Below is an excerpt taken from the transcript of one of Prof Syed Naquib al Attas's lectures:
Lecture 3: 23 Feb 1998, pages 61 to 65

Why we are saying this now is because we have to understand today there are people who are trying to spread the idea that all religions are the same, or that all religions have equal truth and equal validity. That cannot be, there is only one truth - there cannot be two or three different truths. Some of the philosophers, especially the (false) Sufis, might argue that is just our point of view. They might say for example, if you look at this table and if you looked from the top of a room you would say it was rectangular in shape. If you looked at it from the side it would be a different shape and if you looked from another angle it might just be a line with four legs, but all these are the visions of the same table. But this kind of analogy is wrong,we are not dealing with physical things like that, God cannot be viewed in that respect, from different angles as if there are different gods, or different regions of Him.

These people, I am referring to a man called Schuon, who spread this idea of transcendent unity of religion, say as if every religion is not perfect. They say there is no one religion that can claim truth absolutely because each one of them looks at God from different perspectives and therefore the God every religion worships is not God as He is in Himself. (ie. Not God on the absolute sense). From their point of view it is just a limited view if God, and therefore not entirely true - giving the impression as If all religions as a whole might give a true picture but each one of them is only of a limited perspective. This kind of idea we certainly will not accept, the Muslims will never accept that. As long as there are scholars among Muslims who understand these things, they will never accept the idea of transcendent unity of religion.

We say that the conception of God in Islam is derived from revelation not from philosophical thinking. We know He is One, He is called by many different Names, He has attributes and they all refer to Him, and that He in Himself cannot be known and apprehended by His creatures. The Quran says there is nothing like Him, because our human mind usually always tries to make analogies and comparisons, you can't compare. So all Muslims understand that, they do not make images of Him in their imaginations. Muslims at the age of six, when they start reading the Quran are already trained to think of God in an abstract sense and not to confuse Him with His creation. So Muslims from young are already taught that God has no forms that they can imagine, for example, and that He has no "six directions" - there is no above and there is no below, there is no left and there is no right, and there is no front and behind.

Forms and images can only refer to created things, to His creation, not to God. So in this particular respect, whilst we acknowledge God to be such, there is of course God as He is related to His creation. That is why in the Quran the word rabb refers to God when He is related to His creation, there is already creation but there is an aspect of God that is unknown (if one may so) before creation, because there is a before and that before is He. There is a Hadith of the Prophet, when somebody asked him where God was before the creation, he said " God was and there was nothing with Him." Later on al-Juneid was reported to have said that there was an addition to that Hadith: "He is now as He was" - meaning even now there is no witness as far as God is concerned. (* reminds of syahidallhu annahu laa ilaaha illa Huu)

We have to understand that when we regard God as a Creator where He is related to His creation, He is known as Rabb and in that respect, it is only one of His aspects. Therefore Rabb is not God as He is in Himself, it is God, one of His aspects. And then there is another aspect in which He is known as ilah, meaning the God of religion. The transcendent unity group say all religions admit God, they don't deny it. But then this God they admit to, is not the ilah, it is Rabb. it is not the discovery of Schuon because the Quran already says you cannot deny Rabb - that cannot be denied but what can be denied is the ilah part. That is why the kalimah if the Muslims is la ilaha illa Allah, not la rabba ha illa Allah. Questions like these have not been raised before and the answers I have given have also not been given before. Therefore it is important for you to understand these, if not then you have to ask questions. La ilaha illa Allah means there is no God truly worthy of worship except Allah. Rabb of course is accepted by everyone, including Satan. The Quran says that Satan said to the people: "be rejectors of truth" in other words, "be kafir" and after the people became kafir, then Satan said to them: "I am really afraid of Rabb." hence Rabb al-alamiin, that means He cannot be rejected. This means that people who reject God, who reject the true worship and ilah, are still kafir - they have committed kufr as far as we are concerned, even if they believe that He is one and He exists.

End quote.
May we all benefit from such lucid clarifications by Prof!