This question is hard. This is one of the reasons why Rob Bell's Love Wins has been such a conversation starter.
The debate of Inclusivists vs. exclusivists and how many go to heaven has gone on for years.
What exactly is salvation and when are we sure it has happened in a person?
Are Catholics in or out? what about Jews?
It does seem clear in the Bible but is can also vary a little from person to person it seems. We have a pretty good idea that the only way guaranteed by God in the Bible is through faith in the person of Jesus who not only died to take care of our sins but has the power to raise himself and others from the dead.
At the same time, the Bible is clear that God is the only one who can truly read all the hearts of men. So we can do our best to point people to Christ and warn them from the danger we can see on the outside, but always being aware that we cannot see a all of a person and everything about them the way God does.