Essential points:

1. - Earlier revelations, such as the Torah, the New Testament, and sacred scriptures such as the Hindu Vedic Scriptures, or the Teachings of the Buddha have but come to a certain people or limited area at a certain time in history, whereas the message of Islam has been given to mankind in general, everywhere, until the end of times.

2. - Those religions have already reached and passed their zenith, their highest point of realization.
Islam however, despite some recent modernist tendencies, is still a living tradition linking back to its prophetic source and is therefore still valid - and will be - until the Day of Judgement.

3. - What happened to those previous religions is that on the one hand their doctrins or teachings were changed or forged[1] so that they became faulty and diluted – either intentionally – to the detriment of all those believers who sought reliable divine guidance – or on the other hand 'unintentionally' through their neglect of the doctrines when their representatives stopped paying attention to them and stopped living up to those teachings.
Because of the alterations to the original message of their respective revelations, their sanctifying power and Divine blessings to guide man aright diminished more and more, if not disappeared completely.
This is one of the reasons why the message of Islam (accepting the will of Allah by following the pattern of the Prophet ) was revealed to mankind, to restate the original message and to establish an authentic spiritual link from the Messenger of Allah .

4. - What had been revealed through earlier traditions (see above) has essentially become 'absorbed' into the Islamic teaching. The Islamic doctrine is general and encompassing, it is to be applied generally in space and time while its landmark is encompassing Divine Mercy and Compassion (rahmah); it is a confirmation and clarification of what came before (of Divine revelations), answering any existential problem or situation of ancient man as well as of modern man and his society.

5. - Concerning what is sometimes called 'the transcendant unity of religions':
traditional Islam does not use this term, because it can lead to misunderstandings. However it is correct to say that the world-religions do have common 'core principles', because these principles are eternal, from a Divine Origine and therefore according to the Truth. Then these items of faith have to be checked.
They are:
•   Faith in God, or a supernatural eternal Being, in the next life beyond this one, and in holy scriptures.
•  However, the fact that we see these common principles DOES NOT MEAN that all religions are somehow the same or equally valid, they are neither the same nor equally valid. As individual man or woman, one has to live by the last of the revealed religions: Islam, which has - as shown above - superceded the earlier traditions. And it did abrogate those earlier religions because of God saying so Himself [2] and also because of the authenticity of its message or revelation. [3]

Furthermore it is not enough to have a mental attitude of 'knowing' the metaphysical doctrines, without realizing the message individually and personally, ie. without being guided by this 'knowledge' to act accordingly: The Prophet of Allah called Islam Faith, Prayers and actions [for the pleasure of God] [4]. Otherwise it is only question of rational, discursive knowledge, without spiritual advancement, which is worse than not knowing about it in the first place:
‘The intention is that the fulfillment of the Shari`a may lead to the haqiqah (Divine Truth).’

Here we may say this Du`a:
Allahumma thabbit qulûbana `alaa ta`âtika
O Allah! Strengthen our hearts for our obedience to You

6. - The word 'Islam' in sura Âl-`Imrân (3) has of course to be understood also in its ethymological term of surrendering to the Divine Will.

7. - It is not the position of any of the Shuyûkh of orthodoxy (traditional Islam), to claim that non-islamic religions are valid. ‘However it is true that all religions which believe in all prophets and in all the revelations coming to them through the Creator, are the same and the target of all religions is the same, but the methods have changed throughout times.’ (see 5.)

9. - Chittick's 'misunderstanding':

It was also shown by Shaykh Nuh H M Keller [5] “that Chittick's claim above that Ibn al-`Arabi "does not draw the conclusion that many Muslims have drawn -- that the coming of Islam abrogated (naskh) previously revealed religions" is false”, as is seen from the above quote by Ibn al-`Arabi, which Chittick is neglecting. (See chapter 2 at Nuh H M Keller's page [5] ).