بسم الله الرحمن الرحيم
Wa alaykumus salam wa rahmatuLlahi wa barakatuh.
If your question is simply cognitive, the concise answer to it is that anyone guilty of violating the Daruriyyat of Deen is out of the fold of Islam, and all the consequences pertaining thereto would apply to him.
On the other hand, if the question is of a polemical nature, you should know that neither one nor ten fatawa of takfeer will achieve the purpose of stemming the tide of Shi’ism. The four decades that passed since the Iranian revolution of 1979 have seen numerous fatawa of takfeer–yet the problem still persists. This sheer persistence, if nothing else, should suffice to indicate that takfeer alone cannot solve the problem.
Takfeer, furthermore, has a number of negative outcomes associated with it:
• It creates the erroneous impression that “the job is done,” thus obviating the need for detailed investigation and the building of acute expertise. The result, ironically, plays directly into the hands of the Shi’ah.
• Any fatwa of takfeer would only be juxtaposed against another fatwa of no takfeer–of which there are any number to choose from. There is a world of difference between fatwa and ijma’.
• By issuing a fatwa of takfeer one immediately allows himself to be characterized by the opposition as an “extremist” compared to whom the “moderates” are more reliable. Again, this plays into the favour of the Shi’ah rather than against them.
• A takfeeri approach creates natural sympathy for the Shi’ah in some hearts, thus assisting rather than resisting the spread of Shi’ism.
Nothing of the above originates out of a mere sucking of theoretical thumbs. They are lessons learnt from three decades of bitter experience. In Imam Razi’s words:
من جرّب مثل تجربتنا، عرف مثل معرفتنا.
It would be a grave error to reduce the phenomenon of Sunni-Shi’i marriages down to a simple fatwa matter. The fact of the matter is that opinions have differed around the validity of such marriages for centuries before today. Have a look at Mawlana Gangogi’s fatwa on the issue (Fatawa Rashidiyyah p. 426) wherein he takes the approach of saying: Those who regard the Shi’ah as kafir take a certain view, while those who deem them merely fasiq take a different view. Although he sets himself on the first side, he did not completely expunge the other view but duly acknowledged its existence.
Those who issue the fatwa of takfeer will (and, for that matter, should) continue doing so. But they should not make the mistake of believing that a fatwa will solve the entire problem, or that everyone else should of necessity endorse the fatwa.
The polemical rebuttal of Shi’ism require much greater assiduity, refinement and surgical precision than the blunt hammer blow of a fatwa. For ourselves, therefore, we prefer to avoid the takfeeri approach. In the briefest of nuthshells: it simply doesn’t work.