I fear that these people will start quoting the abovementioned incident of Mansoor Hallaaj (rahimahullah) to befool the unwary masses.

Answer

Let us momentarily assume that Mansoor Hallaaj did utter statements which lead to the idea of 'unity of religions'. On the basis of this assumption there is no Muslim whose Imaan is valid and healthy who will accept that the Qur'aan and the Ahaadith can be abrogated/cancelled by an utterance of a man in ecstatic intoxication some centuries after Rasulullah (Sallallahu alayhi wasallam).

Many deviates and hypocrites had dotted the history of Islam. It does not flow from their preachings and concepts of kufr and shirk that the Islam of the Qur'aan becomes cancelled. Only such people who are destined to be fuel for Jahannam will renounce their Imaan and allow themselves to be ensnared into the trap of Iblees. True Muslims believe in the Qur'aan, not in the utterances and concepts of men who appeared after the finalization and perfection of Islam.

The murtad khateeb had promoted his concept of kufr to bootlick the Christian interfaithers. While he may be at pains to cunningly promote Christian shirk and kufr, the Qur'aan Majeed rejects and condemns with much emphasis the Christian concept of trinity. Now how is it possible for Islam to unite with a religion which has the doctrine of trinity (three gods in one) as its pivotal belief. Condemning the Christians, the Qur'aan Majeed says that the Nasaara claimed that Nabi Isaa (alayhis salaam) was the 'son of Allah', and condemning the Yahood, the Qur'aan states that the Jews claimed that Uzair (Alayhis salaam) was the 'son of Allah'.

The Qur'aan has only criticism and condemnation for all religions other than Islam. How can there be 'unity of religions'; when the Qur'aan states with great clarity and emphasis that the mushrikeen will dwell in Hell-Fire eternally?