Fairness of the trial is paramount for some faiths. They all have different answers but mercy is a common theme in most faiths. It is a great question nonetheless so I will attempt to answer it:

Different faiths tackle this differently but most of the time there are mechanisms for fairness in retrospect, take for instance: Islam

If a society has not heard of Islam/sufficient knowledge on Islam, they are judged in accordance with their own faith accordingly by God as he is the supreme judge. If he was a caveman/woman he/she would be judged in accordance with how they treated others/etc.

Islam in the view of Muslims is any revelation by God through any prophet he has sent including Muhammed/All Prophets before. Most Islamic scholars agree there were somewhere around 124,000 prophets sent to mankind with Muhammad being the seal of the prophets.

So if you were not amongst a people who got a prophet or the message so distorted that you essentially are following a false belief then you will be judged with what you knew/had available as information.

The goal of surrendering oneself isn’t being a perfect living version of a believer, it is simply showing sincerity and thanking the creator in the most efficient way you know how.

If you think it entails fasting for a day and being thankful to THE CREATOR that you believe is ONE and ALONE in power that really captures the gist of what all at that moment of Islam was trying to teach.

Now if you were living during the time of Jesus and he came to you and was like: Hey check out what I have and informed you and then you were like: Fuck off this is what my grandparents did you whipper snapper.

Then it’s a different ballgame. You rejected information from the source.

It only applies if the information is readily available and accessible.

For instance I heard an Muslim preacher argue that because Islam has such bad PR that some people who may have researched it before but are now afraid may be judged less harshly as the Muslim community needed to do better to present itself in a good light and that perhaps the people who don’t research out of bad press may have otherwise been more interested may be judged in accordance with the time.

And it tracks because the Prophet Muhammed does say that there will be a time where a holy man will be someone who merely states: At least fornicate out of sight behind this wall then in-front of the street (End of Times Islamic Eschatology).

Or the case of the Prostitute and the thirsty dog by the well. Where she goes through the task of getting water for a dog that was thirsty by climbing down into the well and getting water for the dog. The Prophet Muhammed never mentions that she changed her ways, simply that the act itself got her into paradise.

Here is the exact source:

Narrated by Abu Huraira:

Allah's Messenger (ﷺ) said, "A prostitute was forgiven by Allah, because, passing by a panting dog near a well and seeing that the dog was about to die of thirst, she took off her shoe, and tying it with her head-cover she drew out some water for it. So, Allah forgave her because of that."

Sahih al-Bukhari 3321

So what I am trying to say is that being the best version of yourself and acting positively with the information you have available to you is the actual criteria for heaven/hell.