Once again, according to numerous hadiths that Sunnis have narrated, the condition for the acceptance of good deeds by Allah (swt) is the acceptance of the succession and imamate of Imam Ali (A.S).

The prophet of Islam (PBUH) has said: “Looking at the face of Ali ibn AbiTaleb is worship, remembering him is worship, and no one’s faith is accepted without loving him and hating his enemies.”‌[13] What this hadith is saying is that if one’s faith is to be accepted, its condition is Imam Ali (A.S). True faith circles around him.

Sunni scholars have also narrated the prophet (PBUH) saying:
“O Ali! If anyone spends a course of time the length of Prophet Nuh’s (PBUH) lifetime in worship, and has gold the size of the mountain of Uhud and gives it all away in charity, and has a long life that allows him to perform Hajj a thousand times by foot, and is then wrongfully and innocently killed between Safa and Marwah, yet doesn’t believe in your imamate and succession O Ali! Will never smell the scent of heaven and will never enter it.”‌[14]

Does all of this mean that no one other than the Shia will go to heaven?

Those who weren’t Muslim can be divided into two categories:

1- The infidels and rejecters of faith: Those who knew Islam was the true religion, yet refused to accept it out of negligence and other personal reasons. This group deserve to go to Hell and remain there forever.

2- Those who never heard of Islam or what they heard of was a small part of it, making it resemble other religions that they knew were false. Such people will be saved given they were sincere in their own religions.

Islam says these people will have a chance if they had followed their own religions; religions that were in harmony with their human nature.

The same things that were said go for those who were Muslim, but didn’t hear about Imam Ali (A.S) or heard very little.
In a nutshell, all of those whom the truth hasn’t reached not as a result of shortcoming and not striving for the truth, don’t deserve to go to Hell, because it is the place for the sinners, and those who tried but never found the truth aren't considered sinners and negligent.

For further information, see:

1- Hell and Non-Muslims, Question 47 (site: 283)

2- Attentive ignorant individuals and deliverance from the hellfire, Question 323 (site: 1751)

 [13] Kharazmi, Manaqib, pg. 19 and 212; Ganji Shafe’i, Kifayatul-Taleb, pg. 214.

[14] “ثم لم يواليک يا علي لم يشم رائحة الجنة ولم يدخلها…”‌ Kharazmi, Manaqib, Maqtalul-Husain, Kharazmi, vol. 1, pg. 37.

Source: shafaqna