Why did the followers of the same religion disagree and separate into 73 groups?

As is seen in almost all religions, the followers of Islam separated into different sects. With the expression of Figlalı these sects are the schools of Islam in fields of faith and practice. (1) Mazhab-sect is a word derived from the root Z-H-B and means the way, one sticks to. (2) Prophet Muhammad (peace be upon him) points to the facts of disagreement in the humanity with his statement: The Jews were divided into 71 groups and the Christians into72. And my community will be divided into 73 groups. All of them will be cast into the fire, except one; and this group is the followers of me and my companions. (3) Another statement weak in relating: all of them are in the paradise, except one. (4)

Although there have been much debate over the soundness of the hadith about the 73 groups, for us this separation is a historical reality, and the hadiths about it are statements of this reality. (5)

When we come to the issue of what is the implied meaning of the 73 groups, there are mainly two views about this issue:
The first one: The ones who accept this expression literally.
They recount the groups of faith and complete the 73 groups with the branches of them.
The principles of these: the Hariji, the Shia, Mutazila, Jabriyya, and Murjia. There is no agreement among the people who try to come to the number 73 with these groups and their branches. (6)

The second: The ones who take the expression 73 groups as an implication to multiplicity.
In Arabic, the expressions like 7, 70, and 700 imply multiplicity. For example; the expressions in the some verses and hadiths seven heavens (7), seven sees (8), even if you repent for seventy times, God will not forgive them.(9), Faith is seventy and more branches, (10) show this fact.

Each group see themselves as the group who will be in paradise. (11)

Muvlut Ozler approaches to the expression all of them are in the fire except one. in this way: This implies not being directly in the fire; but not holding the truth which is the way of the Quran and Sunnah-the style of Prophet Muhammad (pbuh)... Prophet Muhammad (pbuh) points out their mistakes in their views. (13)

Shatibi points out about the same subject: The expression of all of them are in the fire, except one is apparently a statement of warning. The state of being eternal in the fire or not is not expressed. Thus, in this statement there is no proof of eternity; because warning with hell can be directed both to disbelievers and to rebellious believers. (14)

Bediüzzaman describes the group of salvation with the following expression the perfect group of the right path. Hence, there will be only one, which will reach the complete salvation; the others will more or less deviate from the right path.

Prophet Muhammad (pbuh) draws a line on the ground, and says, This is the right path. And he then draws other lines to the right and left of this line. These are the other pathsOn each one of these ways, there is a devil inviting to that way. And recites this verse (16): This is my upright path, stick to it. Do not stick to any other so that it may lead you astray. (17)

It is clear that the shortest way between two points is a line. The curves outside of this more or less deviate from this line.

For example; the sect Muattıla, which denied the names of God performed over-simplification; and the sect Mushabbiha performed exaggeration by likening them to those of the creatures. And the Ahl-i Sunnah, which says, God has names which we cannot understand fully, follows the true path.

Likewise, in the issue of fate, Mutazila, which denies fate, fell into oversimplification; Jabriyya, which denies the free will of human, fell into exaggeration. Ahl-i Sunnah saying: There is a divine fate. But man has a small will of his own; so he is responsible. sticks to the right path.

Just like these two examples, the schools comprising the 73 groups, stood on either the pole of exaggeration or that of oversimplification. The right explanation is that of Ahl-i Sunnah.

Sources
1- Fığlalı, Ethem Ruhi, Çağımızda İtikadi İslam Mezhepleri, Selçuk Yay. İst. 1980, s. 1
2 -İbnu Manzur, Lisanu'l-Arab, Daru Sadır, Beyrut, I, 393-394
3- Tirmizi, İman,18; İbnu Mace, Fiten, 17; Ebu Davud, Sünne, 1
4- Aclûnî, Muhammed, Keşfu'l-Hafa, Daru İhyai't-Türasi'l-Arabi, Beyrut, 1351 h., I, 150
5- Bu konudaki görüşler için, bkz. Özler, Mevlüt, İslam Düşüncesinde 73 Fırka Kavramı, Nun Yay. İst. 1996, s. 29-39
6- Bkz. Özler, s. 67-78
7- Bakara, 27
8- Lukman, 27
9- Tevbe, 80
10- Müslim, İman, 58
11- Bkz. Özler, s. 93-95; Fığlalı, s. 5
12- Fığlalı, s. 2
13- Özler, s. 122
14- Şatıbî, Ebu İshak, el-İ'tisam, Daru'l- Kütübi'l- İlmiyye, Beyrut, 1995, s. 413
15 Nursi, Said, Mektubat, Envar Neş. İst. 1993, s. 106
16- Razî, Fahreddin, Mefatihu'l-Gayb (Tefsiru Kebir), Daru İhyai't- Türasi'l-Arabi, XIV, 3; İbnu Kesir, Hafız, Tefsiru'l-Kur'ani'l-Azim, Kahraman Yay. İst. 1985, III, 360; Kurtubi, Ebu Abdullah, El-Cami Liahkami'l-Kur'an, Daru'l-Kütübi'l-İlmiyye, Beyrut, 1993, VII, 90
17- En'am, 153