The first thing a believer should understand is this, God is perfect in his designs. He would not let a generation exist before Jesus and miss the chance of knowing supposed Son of God. Therefore one must ask, why there were millions of people who lived before Jesus Christ, why they never had the chance to know him?
The answer is they did not need to know him, because Jesus Christ was a prophet of God like Adam and all that came later like Jacob and others. The previous generations and Jesus taught the same thing which is Oneness of God, Lord of All Things. He was not a saviour card that you can use after committing so many sins. He was a creature like Adam and Abraham. Specialty of Jesus is that he was born of a virgin mother. He was a great prophet just like Moses and he never claimed divine sonship.
The Bible was forged by scribes and altered in many ways that the identity of Jesus has become unrecognizable. Therefore the answer is Jesus was no different than Moses in prophethood mission. Previous generations, God created many prophets for each of the nations or tribes. 124 thousand prophets were chosen in the long history of humanity.
In Surah al-Zalzalah verses 6-8, God says about the great apocalypse day:
"That Day, the people will depart separated [into categories] to be shown [the result of] their deeds. So whoever does an atom's weight of good will see it, and whoever does an atom's weight of evil will see it."
This brings the question of what is good and what is evil? Even if a person never met a prophet and never heard a divine message, God will judge this person according to his/her deeds.
Good deeds can be listed as:
- Not harming anyone as long as this person does not harm anyone.
- Not eating unclean food like pork.
- Not drinking anything alcoholic or the kind that will make you lose your control over your body. Like drugs.
- Not saying harsh words to your parents.
- Not performing or supporting LGBT
- Not performing fornication
A person can know in their heart what is good and what is bad. Every person would know -except justice- murder is evil. Stealing is evil. Using swear words against people is evil.
If somebody has intention to be a good person in his/her heart, God would help them. Because Omnipresent and Omniscient God Allah Almighty is everywhere. He knows our intentions in our hearts. He know which way we tend to go, the good or evil. Therefore if we accept there are good and evil deeds and we intend to choose what is good, Allah Almighty will help us achieve goodness.