Sheikh al-Islam Ibn Taymiyyah is a very famous Muslim scholar. To understand Ibn Taymiyah we need to understand the time that Ibn Taymiyah came at. Ibn Taymiyah came at a time when many people came into Islam but unfortunately wanted to bring some of the stuff from their old religions into Islam and make a mix. The result of that appeared as groups like the Shi3a & the Sufis and so on. At that time Islam was drifting away and the people started clinging to the things that were added to Islam not to Islam itself. Ibn Katheer in his most famous book "al-bidayah wal-nihayah (The starting and the beginning)" says that deviation and misguidance speed so much that Sunnah and true Islam was hard to find (much worst than today), and he actually gives examples of places and such. He said "and when we come to see the state of the people of Damascus that time, the people there were worst than the polytheist Kafirs like Christians and Sabians. Innovations and Shirk spread so commonly that it was thought to be Sunnah and it was defended vigorously against any kind of reform or return to the sundae."
At these times Ibn Taymiyah came. Obviously, this was around the time of the Crusader presence. Also, you have a significant of the Christian population becoming Muslim or forming certain sects and certain people who were already Muslims were forming certain sects. As one says, you are a product of your environment. A very large percentage of Syrians were Christian and joining Islam and they were bad influences. Ibn Taymiyah attacked the idea of having saints. The idea of saints originated from Catholicism and Christian Orthodoxy. As for Druze (1) and Alawite (2), Ibn Taymiyah call for their persecution since they are worse polytheist than Christians are. The Mamelukes were not interested in the piety of Orthodox Sunni style. They favored the Mystical Dervish styles of the Sufis etc.... Today, with Mamelukes and Fatimite influence, Egypt still has heavy mystical influence and Sufism.
Ibn Taymiyah had one little magnificent idea: Islam was perfect and complete at the days of the companion, a lot of stuff was added to Islam afterwards by Sufis and others, since Islam WAS perfect before then these things we do no need. So, he wanted to revive the understanding of Islam according to what the Companions of the prophet (or al-Salaf al-Salih7) understood. Of course Ibn Taymiyah did not come up with that concept by himself, the concept was started by the prophet who used to say in every Friday that people will come and they will add to Islam, and that these additions are evil and are rejected, and that those who add them will not be forgiven and there will not get reward for their worship. The Companions understood that and they followed it perfectly. The guidelines of the concept are very apparent in all the 4 schools especially the last school (mathhab), the Hanbali which came at a time when deviations and innovations were many so It had a solid and clear methodology in fighting such things, Imam Ahmad Bin Hanbal spent several years in the jails of Bani al-3abbas because of his stand against the deviant mu3tazillah when they claimed that the text of Quran was uttered by Gabriel not by Allah (fitnat khaliq al-Quran).
Anyway as Muslims got deeper and deeper into these deviations, Allah made their situation worse and worse. First Allah gave the shi3a the upper hand on the Sunnis and they founded states in Yemen (Zaidiya), eastern Arabia (Qaramita), Iran (Ismailis and Twelvers) and Egypt (Fatimites). Ibn Katheer says in the same book (al-bidayah wal-nihayah) that this was indeed the long due punishment that the people deserved with their leaving Islam. But the people still insisted on deviation and innovation so Allah sent both the Mongol and the Crusaders as punishments on the Muslims. At such times, came Ibn Taymiyah.
Ibn Taymiyah was a follower of the Hanbali School, his father was the head of that school but he died when Ibn Taymiyah was 17. When Ibn Taymiyah became 20 he was acknowledged to be the head of the Hanbali School by all Hanbali major scholars. The nature of the Hanbali school and the fact that it was the last lead remain away from all the innovation and deviation aground them. Many Hanbalis deviated too but for some time the only groups that slicked to pure Islam were a small group among the Hanbalis and the ever-small group of the students of Hadeeth. The rest were in dissension. Ibn Taymiyah started a revolution. He was sick and tired of all the innovation and deviation aground him, so he started a merciless war against everything that is not Islam yet claiming to be Islam. He attacked many people that the people considered holy like al-Ghazali, Ibn Arabi, Ibn Seena (philosopher from the Mu3tazila (3)) and many others. Ibn Taymiyah was set on living Islam like the Companions understood it and lived it, without the additions that people added and without taking anything away from Islam. He was able with his great ability at speech and great knowledge to gain a huge number of supporters in a short time. Indeed, Ibn Taymiyah enemies had no argument at all, anyone who studies Quran and Sunnah sees that everything Ibn Taymiyah called for was good and everything he fought was bad. The guy was so careful not to do or say anything unless he is sure of the strength of his argument.
Ibn Taymiyah attacked Ibn 3arabi, the famous Sufi that claimed for the first time that Allah exists everywhere and he claimed that Allah and his creations are the same thing. That is clearly in contradiction to Quran and Sunnah and to the understanding of the Companions (Qur'an 67:16). So Ibn Taymiyah declared him a Kafir (non-Muslim) because he attributed to Allah something that we were never told, and that is Kufr by the agreement of the Sunni Muslims. By doing this, Ibn Taymiyah made the Sufis his enemies. Ibn Taymiyah was merciless with the Sufis who instead or worshipping Allah and supplicating to Allah, were worshipping their Awlia' (their leaders) and the graves, yet he was fair to the few Sufis that were righteous at his time like Abdul Qadir al-Jilani (unfortunately those Sufis are almost nonexistent today). Ibn Taymiyah did not care how much that was man respected nor how many followers did he have, he attacked whoever he found deviating and he was extremely frank about it. He even would say that so and so contradicts himself and show how, or comment on the stupidity and silliness of the ideas and methodologies. He was devoted to the truth.
Ibn Taymiyah did not care who liked him and who hated him, he cared for what was right and what was wrong. Later on a person from 7Hama city asked Ibn Taymiyah about the issue of Allah being over the throne (al-istiwa') so in answer Ibn Taymiyah dictated his book "al-3aqeedah al-hamawiyah" which basically was based on what the Companions believed in regard to theme issues, and he gave extensive evidence for it from the sayings of the companions, the understanding of companions to Quran and from the narrated Sunnah. The fact was that at that time people in Sham (Syria) had a different view on this issue than the view of the Companions, their understanding was that to deny the attributes of Allah and make them symbolist to some other things. Ibn Taymiyah was able to clearly show how this is not what the Muslims for the first three generations believed in, and that this belief was added to Islam. By this Ibn Taymiyah also gained the asha3irah as another enemy. Then Ibn Taymiyah made his most famous Fatwa (religious verdict) with regards to promising to make divorce that those who promise "3alai al-talaq" not intending to really do, then it doesn't count. The three other schools of Fiqh (Islamic jurisprudence) were in agreement on the contrary so they were so raged at his Fatwa, although" today all Muslims are in agreement that his Fatwa was indeed correct. But anyway the result of his Fatwa is that the leaders among the other schools became all against him. So, Ibn Taymiyah gathered against him the ash3aris, Sufis and the Sectarian and that meant trouble and conspiracies to come.
The 3 groups started making plans to get rid of Ibn Taymiyah from his position, so they went to the ruler and told him that Ibn Taymiyah claims that Allah is in the sky and that this is bad. The ruler was fair and held a debate between both sides: Ibn Taymiyah, all the judges, and all the others. At the end of the discussion the ruler said that Ibn Taymiyah was correct and that these scholars were misguided about where Allah is (ola'ika ada3u rabbabum).
Issues like this continued and later the 3 groups managed to get other rulers to be less sympathetic with Ibn Taymiyah and he was jailed several times. That is no shame of course since Imam Ahmad himself was put in jail and prophet Yusuf (Joseph) was put in jail too. Ibn Taymiyah also spent some of his time in Jihad against the Mongol. He also gave several motivating speeches to the armies that went to fight when he did not go with them himself.
Ibn Taymiyah, with his daring frank approach was very successful at exposing the zeal and deviation of many groups. He was one man who started with a few students but ended up with a huge group of followers returning to follow Islam like the Companions did. The prophet told us about such people when he said "Allah has put for my nation for every 100 years a man who will revive the forgotten and correct the deviation." and Insha' Allah Ibn Taymiyah is among those Reformers (Mujadids).
The efforts of Ibn Taymiyah soon lead to a righteous government by the Ayyubis to be formed. Bani Ayyub (children of Saladin/Salah al-Deen) adopted an Islamic reform policy that they are until today praised for, they followed much of what Ibn Taymiyah called for with a few exceptions, the most important one was that they were still ash3aris in their faith. But the fact that they were the only Sunni righteous government for a long time, and that they declared their Sunnism and a war against so many deviations shows that they are indeed a gigantic improvement over those who came before them.
As for his Fatwa regards the Isma3ilia (Isma'ilism) being Kafirs (non-Muslims), then that is true. He did declare them Kafirs, but then again so do ALL Muslims. The Isma3ilis believe Quranic texts have a hidden mending, and that there is a knowledge of the unseen, that only the Ismaili scholars can know about. That is known as Batiniyah (hidden religion), and the Sunni Muslims are in 110% agreement that this is s Kufr (disbelieving in Islam). Besides the fact that Ismailis believe that Quran is distorted, this is Kufr too. As for his Fatwa that the ruler should kill them all, then that is also not his opinion alone. Many scholars since the days of the Companions believed that a Zindeeq (the non-Muslim who claims himself to be a Muslim to attack Islam from inside) has no repentance in this life, that we must kill him, if he is under Islamic law. And this is by the way, the opinion of the majority, but the Ismailis hate Ibn Taymiyah so much because only the students of Ibn Taymiyah are the ones that refute these groups and expose their Kufr to all the people so that they will not be fooled by them.
Ibn Taymiyah died young but he left a lot of books. Many of Ibn Taymiyah students became big time scholars like Ibn al-Qayim and al-Thahabi. Ibn Taymiyah efforts are until today very useful, his guideline are until today used by the Muslims who want to follow Islam like it was at the days of the companions, without any additions and without any deletions.
Shaikh al-Islam Ibn-Taymiyya was the best when it comes to debating with Christians. If you read his debate with Christian scholars, you will find him supper. He showed very clearly their contradiction and man-made modification to their Bible. His book "Al-Jawab Al-Sahih Liman Baddala Qaula al-Maseeh (The right answer to those who changed the message of the Messiah)" is the best ever book that was written against the missionary Christians. May Allah bless his soul, he was a great man. I have to say that all Damascus attended his funeral including all these who he was in-dispute with them about some branches in Islamic practices like the Sufis and other schools. Muslims prayed at him even in far countries such as Yemen and China. Ibn-Taymiyah was one of the greatest men in Islamic teaching he is one of very few Imams who fought on the front-line side by side with Muslim soldiers. He was school in Fiqh and Jihad at the same time.
See also: [The relationship between Mohammed bin Abdul-Wabhab and Ibn Taymiyah]
1. The Druze supposedly the progeny of the Shiite faith from the Fatimid Empire that Saladin Al Ayubbi destroyed. They belive that al-Hakim (985-1021), an Ismaili ruler, is the embodiment of God.
2. But what about Ibn Taymiyah's supposed call for the killing of Alawites, because they are not following the Sunnah way. It is known that the Alawites and Druze both have some aspects of Islam, Christianity, Judaism, and pre-Christian and pre-Islamic philosophy on spirituality. Even Christians had pre-Christian traditions in their religion, so it was natural when some converted that they brought some of that.
3. As far as the Mu3tazillah they are a split from Islam that tried to have certain Neo-Platonic ideas and have Greek Philosophy melange with Islamic ideas and have esoteric ideas.