If someone doesn't know about Christianity, and they sin, do they go to hell? I was debating with a friend recently and he is from India, he never knew about Christianity until he came to the US, and he made a sin, will he go to hell?
There are lots and lots of different positions on this matter, this link is a very good summary.

To summarise even further the points in the article:

1. Church Exclusivism - outside the church there is no salvation (therefore there is no salvation for unreached people)

2. Gospel Exclusivism - outside the gospel there is no salvation (therefore there is no salvation for unreached people)

3. Special Revelation Exclusivism - outside special revelation there is no salvation (special revelation includes the gospel/the church, but also includes dreams/visions/direct words of God to non-believers and unreached people)

4. Agnosticism - neutral position, committed only to not knowing

5.  General Revelation Inclusivism - anyone can be saved by general revelation (belief in a non-religion specific God) but other religions are in no way instrumental to salvation

6.  World Religions Inclusivism - similar to (5), but also holds that other religions can be used for salvation.

7.  Postmortem Evangelism - unbelievers who die will have a chance to trust Christ after death

8.  Universalism - everyone will be ultimately saved by the Christian God

9.  Pluralism - everyone will be ultimately saved by God, the Christian image of which is no more or less valid than that of other religions

There's also a tenth position I'm aware of, but I can't find anybody willing to give it a name:

10. Universalism for the unreached - all unreached people will be saved, God only rejects those who reject him first

This is a very problematic position though, because it begs the question, why evangelise in the first place, given that it only decreases one's chance of salvation.