Shaykh Al-Islam ibn Taymiyah wrote:
“Those who excommunicate the exponents of heresy and fancy (ahl al-bid’ah wa al-Ahwa) like the Shi’ah, the Mu’tazilah and others, go against the Qur’an and the Sunnah, the consensus of the Companions and the Successors.
The proof that the Companions did not declare the Khawarij kafir is that they offered salah behind them. ‘Abdullah ibn ‘Umar and other Companions, for example, offered salah behind Najdah Al-Haruri. They discussed with them various issues, gave them their opinions on religious matters put to them, and addressed them as a Muslim addresses another Muslim. ‘Abdullah ibn ‘Abbas, for example, would answer the questions which Najdah Al-Haruri sent to him. Al-Bukhari has recorded the hadith which Najdah has related. Similarly, Nafi’ ibn Al-Azraq answered many of his questions, and used to argue with him on different issues quoting from the Quran just as he did with any other Muslim. This was the practice of Muslims throughout the ages; they did not consider them apostates like those against whom Abu Bakr declared war. They did this even though the Prophet (saw) had commanded to fight them, as is reported in authentic ahadith. The ahadith that they are the worst people under the sky to be killed and that the men whom they kill are the best men, which Abu Umamah has reported and At-Tirmidhi has recorded, only means that they are more harmful to the Muslims than others. And it is true that they have done greater harm to the Muslims than the Jews and the Christians. They were zealous to kill any Muslim who did not agree with them, since they believed that it was quite lawful for them to take his life and property or kill his children, for he was a kafir in their view. They considered it to be an act of devotion, as they were immersed in ignorance and heresy. But in spite of that the Companions or their righteous Successors did not declare them kafir or apostate (murtadd), nor did they cause them any wrong in word or deed. They feared God with regard to them and were never unfair to them.
This was their practice with all the other exponents of bid’ah, the Shi’ah, the Mu’tazilah, and others. Hence, if anyone declares kafir any of the seventy-two sects, he goes against the Qur’an, the Sunnah and the Consensus of the Companions an their righteous Successors. It may also be noted that the hadith which speaks of the seventy-two sects does not occur in the Sahihayn. Ibn Hazm and others consider it to be weak, other rate it as hasan, still others like Al-Hakim rate it as sahih though the compilers of the Sunan collections mention more that one channel through which it has been transmitted.
Furthermore, the words of the Prophet (saw) “seventy-two of them will go to Hell and only one will be saved” are not stronger than many words of God such as “Those who unjustly eat up the property of the orphans eat up a fire into their own bodies, they will soon be entering a blazing fire” (4:10) or “If any do that in rancour and injustice soon shall We cast them into the Fire, and easy it is for God” (4:30) There are many such verses that threaten with Fire those who commit evil deeds, but they do not do so WITH REGARD TO ANY PARTICULAR PERSON, FOR IT IS POSSIBLE THAT HE MAY REPENT OR THAT HIS GOOD DEEDS MAY OUTWEIGH HIS EVIL DEEDS, OR THAT GOD MAY WASH OUT HIS SIN BY SUBJECTING HIM TO ONE OR ANOTHER TRIBULATION.
(Minhaj as-Sunnah 3:62) as quoted in “Ibn Taymiyyah Expounds on Islam” by Muhammad Abdul Haqq Ansari p 557-558