All the followers of the various sects of the this Ummah will be    in Hell apart from one
Question

                I would be very grateful if you could advise me on the fundamental differences between the main sects of Islam, although I know that originally Islam was a unified religion.
Thankyou.
Praise be to Allah.The answer to your question is lengthy and deserves a book to itself. We cannot answer in details here, but we can say the following in brief:
The Prophet (peace and blessings of Allaah be upon him) told us beforehand that this Ummah would be divided as the previous nations were divided, as stated in the saheeh hadeeth:
Wahb ibn Baqiyyah told us from Khaalid from Muhammad ibn ‘Amr from Abu Salamah from Abu Hurayrah who said: the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “The Jews were divided into seventy-one or seventy-two sects, and the Christians were divided into seventy-one or seventy-two sects, and my Ummah will be divided into seventy-three sects.” This was narrated by Abu Dawood in his Sunan, Kitaab al-Sunnah, Baab Sharh al-Sunnah.
It was reported from ‘Awf ibn Maalik who said: the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “The Jews were divided into seventy-one sects, one of which is in Paradise and seventy are in the Fire. The Christians were divided into seventy-two sects, seventy-one of which are in the Fire and one is in Paradise. By the One in Whose hand is the soul of Muhammad, my Ummah will be divided into seventy-three sects, one of which will be in Paradise and seventy-two will be in the Fire.” It was said, O Messenger of Allaah, who are they? He said, “Al-Jamaa’ah.”
Sunan Ibn Maajah, no. 3982.
What is meant by al-Jamaa’ah is the ‘aqeedah and actions of the Prophet (peace and blessings of Allaah be upon him) and his Companions.
Among the sects that claim to belong to Islam are those who are misguided with regard to the Unity of Allaah (Tawheed) and His Names and Attributes; they say that everything that exists is Allaah and that He is incarnate or present in His creation – glorified be He far above what they say. On the contrary, He is above His heavens, settled on His throne, distinct from His creation.
Others are misguided with regard to eeman (faith) and say that deeds are separate from faith and that faith does not increase or decrease. The correct view is that faith consists of words and deeds; it increases when one does acts of obedience to Allaah and decreases when one commits sin.
Others are misguided by their declaring that the person who commits sin goes beyond the pale of Islam and is doomed to eternity in the Fire. The correct view is that the one who commits a major sin – apart from shirk and al-kufr al-akbar – does not go out of Islam.
Others are misguided with regard to the issue of al-qadaa’ wa’l-qadar (divine decree), and said that man has no choice in what he does. The correct view is that man has freedom of will on the basis of which he will be called to account and will bear the consequences of his deeds.
Others are misguided with regard to the Qur’aan, and say that it is created. The correct view is that it is the words of Allaah, revealed not created.
Others are misguided with regard to the Sahaabah, whom they revile and denounce as kaafirs even though they are the companions of the Prophet among whom the Wahy (Revelation) was revealed, and they are the most knowledgeable and the most devoted in worship among this Ummah, and they strove in jihaad for the sake of Allaah and Allaah supported this religion through them – may Allaah be pleased with them all.
And there are other groups who deviated from the path of Islam and innovated matters in the religion of Allaah; each of them rejoices in what they have, but they have followed the ways of the Shaytaan and gone against the words of Allaah (interpretation of the meaning):
“And verily, this is My straight path, so follow it, and follow not (other) paths, for they will separate you away from His path. This He has ordained for you that you may become Al-Muttaqoon (the pious)” [al-An’aam 6:153]
We ask Allaah to make us among the followers of the Sunnah who will be saved from the Fire, and to admit us to Paradise among the righteous. May Allaah bless our Prophet Muhammad.