Shaykh al-Islam Ibn Taymiyyah stated in Majmu' al-Fataawaa (3/356), summarizing the essence of the saying of the Raafidah Shi'ah:

أصل قول الرافضة : أن النبي (صلى الله عليه وسلم) نص على علي نصاً قاطعاً للعذر ، وانه إمام معصوم ومن خالفه كفر ، وأن المهاجرين والأنصار كتموا النص وكفروا بالإمام المعصوم ، واتبعوا أهواءهم وبدلوا الدين وغيروا الشريعة وظلموا واعتدوا ، بل كفروا إلا نفراً قليلاً : إما بضعة عشر أو أكثر ، ثم يقولون : إن أبا بكر وعمر ونحوهما ما زالا منافقين ، وقد يقولون : بل آمنوا ثم كفروا ، وأكثرهم يكفر من خالف قولهم ويسمون أنفسهم المؤمنين ومن خالفهم كفاراً ، ويجعلون مدائن الإسلام التي لا تظهر فيها أقوالهم دار ردة أسوأ حالاً من مدائن المشركين والنصارى ، ولهذا يوالون اليهود والنصارى والمشركين على بعض جمهور المسلمين ومنهم ظهرت أمهات الزندقة والنفاق ، كزندقة القرامطة الباطنية وأمثالهم ، ولا ريب أنهم أبعد طوائف المبتدعـة عن الكتاب والسنـة ، ولهذا كانوا هم المشهورين عند العامة بالمخالفة للسنة ، فجمهور العامة لا تعرف ضد السني إلا الرافضي فإذا قال أحدهم : أنا سني فإنما معناه لست رافضياً ، ولا ريب أنهم شر من الخوارج ، لكن الخوارج كان لهم في مبدأ الإسلام سيف على أهل الجماعة ، وموالاتهم الكفار أعظم من سيوف الخوارج ، فإن القرامطة والإسماعيلية ونحوهم من أهل المحاربة لأهل الجماعة ، وهم منتسبون إليهم ، وأما الخوارج فهم معروفون بالصدق ، والروافض معروفون بالكذب ، والخوارج مرقوا من الإسلام وهؤلاء نابذوا الإسلام

The foundation of the saying of the Raafidah (is): That the Prophet (sallallaahu alayhi wasallam) textually affirmed Ali [as rightful successor to leadership] with a definitive text that provides the reason (excuse) [as to why he did not actually receive it]; that he (Ali) is the infallible imaam; that whoever opposes him has disbelieved; that the Muhaajiroon and the Ansar concealed the text and disbelieved in the infallible imaam, followed their desires, altered the religion, changed the Shari'ah and oppressed and transgressed, rather, that they all disbelieved except a small faction, either ten-odd or more. Then they say: "That Abu Bakr, Umar and their likes never ceased being hypocrites," and sometimes they say: "That they believed, then disbelieved." Most of them (the Raafidadh) declare as disbelievers those who oppose their saying. They call themselves believers and those who oppose them as disbelievers. They treat the cities of Islaam in which their saying has not been proclaimed (openly) as being abodes of apostasy, being of more evil condition than the cities of the Pagans and Christians. For this reason they ally with the Jews, Christians and Pagans against the generality of the Muslims. From them (the Raafidah) appeared the foundations of heresy and hypocrisy, such as the heresy of the Qaraamitah Baatiniyyah and their likes. There is no doubt that they are the furthest of all the factions of the innovators from the Book and the Sunnah. For this reason they are well-known amongst the general-folk as having opposition to the Sunnah. For the generality of the common-folk do not know of an opponent to a Sunni other than a Raafidee, and when one of them says, "I am a Sunni", then it's meaning is "I am not a Raafidee." There is no doubt that they (the Raafidah) are worse than the Khawaarij, even if the Khawaarij, at the early part of Islaam, raised the sword against the people of the (united) jamaa'ah, and their loyalty (that of the Raafidah) to the disbelievers is greater than that of the swords of the Khawaarij. For the Qaraamitah Ismaa'eeliyyah and their likes are from those who wage war against the people of the jamaa'ah (of sunnah), and they ascribe themselves to them (the Raafidah). But as for the Khawaarij, they are known for truthfulness whilst the Raafidah are known for lying. The Khawaarij exited from Islaam whereas these ones (the Raafidah) resisted Islaam [from the outset].