What are Post-Islamic Religions?

Answer 1

Definition
Post-Islamic Religions are typically defined as religions that developed as outgrowths of Islam that changed and developed one or more of the core teachings of Islam to the point where they are often considered heresy by mainstream Muslims and will often declare themselves as a distinct faith outside of Islam. However, these religions share most of Islam's fundamental teachings and consider the Qur'an a holy book. Since these religions tend to have few adherents and those adherents stay primarily in the Middle East, they are often overlooked by people who talk about the five major religions: Judaism, Christianity, Islam, Hinduism, and Buddhism. Even combined, they represent less than 25 million people.

Discussion of various Post-Islamic Faiths
The Post-Islamic Faiths include Baha'i, Druze, Azali, Yezidi, Yarsan, Alawite, Alevi, and Ahmadiyya. Some of these religions are occasionally considered Islamic, especially the latter three, but more often they are not. They are listed below roughly according to when they developed.

Alawites
Of them the oldest is Alawite, which was founded by the Eleventh Shiite Imam in the 10th century. The Alawites have been accepted into the Islamic fold by various fatwas issued by leading Shiite Imams, but the issue remains as to what they believe. There are numerous tenets of Alawite teaching that disagree with Islamic mainstream practice, such as Christian holidays (the Nativity and Palm Sunday) and reincarnation, but what causes them to be outside of Islam is that they believe that Ali (son-in-law of Mohammed) has a semi-divine status.

Druze
The next oldest is Druze, which developed in the 11th and 12th centuries. The first Druze were Ismaili Shiite Muslims that were influenced by Greek philosophy and Gnosticism. This resulted in them having doctrines about the nature of God's appearance in the physical world and a unique holy book: Kitab al-Hikma and have superseded the five pillars of Islam with the seven precepts of the Druze.

Alevis
Alevis were a derivative of Alawites who mixed in Persian and Mongol traditions in a mystical way. It is unclear when the Alevis developed exactly, but it is likely in the 13th and 14th centuries.

Yezidi & Yarsan
The Yezidi and Yarsan developed in Persia in the 12th and 14th centuries respectively with the Druze and come under the combined header of Yazdanism. These religions are based on a mix of Persian and Zoroastrian beliefs on the one hand and Sufi mystical Islamic beliefs on the other hand. These religions prevailed almost exclusively among Kurds. The key difference between Yezidi and Islam is that Tawûsê Melek, akin to Iblis/Shaytan in Islam, refused to submit to God by bowing to Adam and should be revered for this. Accordingly, Tawûsê Melek remains the leader of the archangels and a source of guidance and inspiration, worthy of worship in addition to God. This is violation of the Islamic precept of "Shirk" that God should have no equivalence. The Yarsan believe that God has become physically incarnate over the generations like Vishnu in Hinduism and that one of these incarnations was Ali (the son-in-law of Mohammed).

Azali and Baha'i
Azali and Baha'i are both descended from Babism, which held that the Bab, a man who lived in the 19th century was predicting the coming of the Mahdi in his lifetime. The Azalis, who were the minority, believed it to be Subh-i-Azal and the Baha'i, the majority, believed it to be Baha'ullah. In either case, their claim rests upon the Bab's ability to have prophecy, which violated the mainstream Islamic belief that Mohammed was the final prophet.

Ahmadiyya
Ahmaddiyya is the most controversial of these groups since Ahmadis continue to argue that they are Muslims (as opposed to most of these groups which do not make that claim). It is based on the teachings of Mirza Ghulam Ahmad, a Pakistani who lived at the end of the 19th century and claimed to be a prophet foretelling the end times and bring about the final triumph of Islam as per Islamic prophecy. He claimed that he was the promised Messiah and Mahdi awaited by Muslims. This also violates the finality of Mohammed's Prophecy

Note on Nation of Islam
It is worth noting that Nation of Islam is not a Post-Islamic Religion, but an African-American religion. The African-American faiths strike a common tone, typically of black supremacy either by birth or by merit and use the texts of pre-existent religions to buttress those claims. This is visible in the Black Israelite Churches, Rastafarianism, and the Nation of Islam. It is for this reason that these faiths bear little in common with the non-African-American faiths that use the same holy texts.