What happens to the souls of people who died before Christianity came about?

Many forms of "heaven" existed and were associated with many of the gods we have worshipped through the ages. The christian concept is not a new one by a long shot!

the story goes according to islam and christianity that god gave many prophets to spread his word and that his words where changed and corrupted time and time again. till he sent his final prophets which christians say ended with christianity and it was the final unchanged word of god and muslims believe it was changed so muhhumad was the final prophet. so as for their souls the ones that followed his message as it was revealed to them would still go to heaven. they where still following the word of god untill it was changed

God's salvation has always been by grace through faith. Just look at the faith of Abraham.

Many went to Hell, many went to hell. Smaller case hell is a waiting place. Similar to the concept of limbo. But limbo doesn't exist.

their judged by how they lived.

they will go to heaven or hell

romans 2:14-16

Jesus has paid for their sins too. If they had faith and good will and good deeds, they are saved.