Fauken:
We can’t assume those few Buddhists, Muslims, and Atheists heard of Christ, which is why he said that. They can’t be condemned for failing to live according to the Gospel when they’ve never heard of it for no fault of their own.
They certainly can and will. If you read through Romans 1-3, you will see that both the Gentiles who were ignorant of the law, and the Jews who had the law were both alike condemned under the law (see Romans 2:12 - but read the line of logic leading up to there to get the reference in context). Then proceed to Romans 3:21-28 where we are justified through faith in Christ.
The idea that those who have not heard the gospel are not responsible for their sin is no where in the Bible. In fact, Romans 10, the same place that provides the means by which we are saved that I mentioned above, also urges believers to be about the business of proclaiming the gospel for just that concern, that those who don’t hear the gospel cannot believe and cannot be saved. Again, this is basic Christian doctrine.