Why do Shi’ah believe that the majority of the Prophet’s companions went astray after he passed away. I sometimes wonder how the mission of the Prophet (PBUH) can be considered a success, if he only managed to convince his family and handful of companions to follow the right way and left the majority of his community to become disbelievers. I find that very disturbing.

This matter is better understood when one considers the following:

Companions who went astray:
This belief is not restricted to the Shi’ah. This is also the Sunni understanding recorded in the most authentic Sunni compilation of ahadith. An authentic hadith about the companions is:
Bukhari narrates from Abu Hurairah who refers to the Prophet (PBUH) saying, ‘On the Day of
Judgement I will see a group of my companions whom, when I have recognised them, will be taken to Hell. If I ask what is wrong with them, the answer will be “They went astray after you … and this continued until only a few remained.” ’

Sahih al-Bukhari, Vol. 8, Bab Al- Hawz, p. 120.

You wonder how the Prophet’s mission can be considered successful, as only a handful of companions and his family followed the right way.
Please note the following:
In the last ayah of Surat al-Jumu’ah, one of the last surahs revealed to the Prophet (PBUH), Allah, refers to the believers mentioned in ayah, nine of the same surah saying,

And when they see some bargain or pastime, they are drawn toward it and
leave you standing.
Qur’an 62:11

1. If this is how Allah describes those around the Prophet (PBUH), what more can anyone else say on the subject?

2. There is no need to concern yourself about the success of Muhammad’s Prophetic mission. It was secured by his progeny Ahl al-Bayt.

3. The Prophet Nuh (A.S) spent 950 years among his followers, and only a few listened to him.

4. The Qur’an describes the majority as going astray, and the minority as remaining on the right track.

Qur’anic reference regarding the majority:
For Allah is full of bounty for human beings, but most of them are ungrateful. Qur’an 2:243

Most people will lead you away from Allah in pursuing their own erroneous
deductions and opinions
. Qur’an 6:116
Say: Knowledge thereof is Allah’s alone. But most people do not know. Qur’an 7:187

Have no doubt, [the Qur’an] is the Truth from your Lord,
but most people do not believe!
Qur’an 11:17

Allah maintains complete control over His affairs; despite most of human
beings remaining unaware of this. Qur’an 12:21

I follow the ways of my fathers, Ibrahim, Ishaq and Yaqub; none of us
attributed any ‘partners’ to Allah. This is His grace to us and to humanity;
but most people are not grateful.
Qur’an 12:38

Yet the majority of humanity will not have faith, however much you might
wish them to.
Qur’an 12:103

In this Qur’an, We have displayed to humankind every [kind of] example yet
most people deny it and refuse to accept it.
Qur’an 17:89

Qur’anic reference regarding the minority:
Then you turned back, all but a few of you, and [even now] you still slide back. Qur’an 2:83
But only a few believed with him. Qur’an 11:40
But few of my servants are grateful. Qur’an 34:13

Most partners [in business] cheat each other except for those who believe and do good deeds and these are few. Qur’an 38:24
Dear friend, do not be disturbed by these facts, rather, pray for Allah to strengthen your faith.