Romans 2:12-16 12For all who have sinned without the law will also perish without the law, and all who have sinned under the law will be judged by the law. 13For it is not the hearers of the law who are righteous before God, but the doers of the law who will be justified. 14For when Gentiles, who do not have the law, by nature do what the law requires, they are a law to themselves, even though they do not have the law. 15They show that the work of the law is written on their hearts, while their conscience also bears witness, and their conflicting thoughts accuse or even excuse them 16 on that day when, according to my gospel, God judges the secrets of men by Christ Jesus.
Now, God is certainly going to be aware that not everybody had the opportunity to hear about Christ. This passage above by Paul, seems to address the issue.
There are a couple considerations.
One is that God says that there will be people from every tribe, tongue, and nation. One thing that is commonly held to be true by Christians is that there is an age of accountability, at which a person becomes accountable for their sin that they willfully committed. That indicates that there is an age at which sin is not credited to the individual. That age varies as a matter of opinion. So a child that dies before a certain age, will be saved, covered by Christ's atonement.
As a result of that, what with the death rate among children in some cultures, an awful lot of "tribals" will be in heaven. It's only the few people who make it into adulthood who have the decision to make whether to follow God or not. CS Lewis makes that observation in, I believe, The Screwtape Letters.
The other thing is that this verse seems to indicate, is that those who follow God's Law from their conscience will be saved.
My thought on this, and this is just a conclusion that I've come to and others will no doubt disagree, is that God will judge them based on whether they would have accepted Christ, had they heard about Him here on earth.
Anyone who truly desires to follow God will not be cast out. If they have never had a gospel witness of Jesus' death and resurrection for their sin, that is hardly a matter over which they have control.
God is just and will judge justly. That doesn't mean the way that everyone thinks He should but with true justice; impartially, without playing favorites.
That certainly comes close to those of the houshold of "faith".... who though they had not seen... they nevertheless looked ahead for His coming.
I also think there are various levels of resistance to the promptings of God as He draws men to Himself. He does move the soul of a person but some get sidetracked along the way...they cannot identify the stirring in themselves... and when God meets them thru whatever means He might use, to take them further along the path to Himself, they deny or resist Him. Human nature does not change...it is in a state of rebellion against any authority higher than itself.
Also ...Truth is hard to accept when one has lived years believing and perpetuating a lie. Man's ego is a powerful thing..and pride the downfall of many a man. We live in a time of a great battle for the souls of men...neither side will be letting go easily...but one or the other will indeed win that soul....
The decision ,critical for all eternity, is man's choice what He will or not believe. And the "authority" he's been given to make that choice is His alone....on this side of eternity.... only.