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حرمة الغيبة مشروطة بالايمان: قوله: ثم ان ظاهر الاخبار اختصاص حرمة الغيبة بالمؤمن. أقول: المراد من المؤمن هنا من آمن بالله وبرسوله وبالمعاد وبالائمة الاثنى عشر (عليهم السلام)، اولهم علي بن أبي طالب (عليه السلام) وآخرهم القائم الحجة المنتظر عجل الله فرجه وجعلنا من أعوانه وأنصاره، ومن أنكر واحدا منهم جازت غيبته لوجوه: 1 – انه ثبت في الروايات (1) والادعية والزيارات جواز لعن المخالفين، ووجوب البراءة منهم، واكثار السب عليهم واتهامهم، والوقيعة فيهم اي غيبتهم، لانهم من اهل البدع والريب (2). بل لا شبهة في كفرهم، لان انكار الولاية والائمة (عليهم السلام) حتى الواحد منهم والاعتقاد بخلافة غيرهم، وبالعقائد الخرافية كالجبر ونحوه يوجب الكفر والزندقة، وتدل عليه الاخبار المتواترة (3) الظاهرة في كفر منكر الولاية وكفر المعتقد بالعقائد المذكورة وما يشبهها من الضلالات

Reference: Mishbahul Fuqahah 2/11, Dar Al-Huda, Beirut Libanon
Online Reference From Shia website:
http://www.yasoob.com/books/htm1/m001/05/no0507.html

Translation:
[I say: What is meant by "believer" here is the one who believes in Allah and his messenger and the last day and the twelve Imams (as), starting with `Ali bin abi Talib (as) and ending with al-Qa'em al-Hujjah, the awaited one may Allah hasten his appearance and make us among his supporters, and he who denies one of them then it is allowed to backbite him for several reasons:
1- It has been proven in the narrations and Ziyarat and supplications that it is permissible to curse the ones who oppose us, and that it is obligatory to disown them, and increase their insults, and accuse them, and slander them, meaning to backbite them because they are from the people of innovation and doubt. There is no doubt about their Kufr, because denying al-Wilayah and the Imams (as) even if just one, and to believe in the Khilafah of others, and to believe in myths such as al-Jabr and other beliefs necessities Kufr and Zandaqah, as mentioned in the Mutawatir narrations that clearly demonstrate the Kufr of the rejector of al-Wilayah, and the Kufr of the one who believes in the mentioned beliefs and similar other misguided beliefs.
2- Those who oppose us commit Fisq publicly, because their deeds are annulled by default as stated by the countless narrations. They even adopted what is greater than Fisq as you now know, and we will mention later that it is permissible to backbite the one who commits Fisq publicly.
3- What we benefit from the verse and the narrations is that it is forbidden to backbite against the believing brother, and it is obvious that there is no brotherhood or sanctity between us and those who oppose us.
4- It is famous and spread since the past among the laypeople of the Shia and their scholars that they used to backbite against the ones who oppose us, rather they used to insult them and curse them at all times and in all lands, in fact it is mentioned in "al-Jawahir" that this is from the necessities.]

After we’re done, now let me show you Taqqiyah:

Imam al-Khu’i's charity foundation called for a conference entitled “الوحدة الإسلامية والمذاهب الفقهية” “Islamic unity and schools of jurisprudence”:

كلمة سماحته في ندوة الاجتماع الدولي لوضع استراتيجية مشتركة للتقريب بين المذاهب الإسلامية المنعقد في فندق إيبلا الشام بدعوة من مؤسسة الإمام الخوئي الخيرية خلال الفترة 10 – 12 /4/1999م

This was held in Damascus 10/12/1999.