The Bible makes a couple of references that apply to this… the most up front are the ones, like in Romans 1 that indicate that God “introduces” or reveals Himself somehow to everyone in His creation, including through the rest of His creation – what is called “firmament” in the King James Version (though I don’t like KJV that much myself).
His creation is then judged based on how they respond to what they know of Him, so that they are “without excuse.”
Historically, we see some of this now – like some Native Americans, even in the midst of their polytheistic tribal religions worshipping the “Great Spirit? – their idea of a creator god; like the Greeks reserving a structure to the “Unknown god,” etc.  Some of these examples can be found in the book “Eternity in Their Hearts.”
Rom 1:18-24
18 For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who suppress the truth in unrighteousness, 19 because what may be known of God is manifest in them, for God has shown it to them. 20 For since the creation of the world His invisible attributes are clearly seen, being understood by the things that are made, even His eternal power and Godhead, so that they are without excuse, 21 because, although they knew God, they did not glorify Him as God, nor were thankful, but became futile in their thoughts, and their foolish hearts were darkened. 22 Professing to be wise, they became fools, 23 and changed the glory of the incorruptible God into an image made like corruptible man–and birds and four-footed animals and creeping things.

Second, and most important is the understanding that a lot of times this question is asked this way: “Where does the (put in the individual of the culture of choice here) that has never even heard about Jesus go when they die”?
In some ways, it is very tough for us humans to give a certain answer – it would be case-by-case, and only God can really know!  The reason is given to the prophet Samuel in the book 1st Samuel “because God judges the heart, while man can only judge the outside.
1 Sam 16:7
7 But the LORD said to Samuel, “Do not look at his appearance or at the height of his stature, because I have refused him. For the LORD does not see as man sees; for man looks at the outward appearance, but the LORD looks at the heart.”

As humans, we are terrible judges because we have our own agendas, biases, opinions, and just actual evil infecting and distorting our perspectives.
God does not, and this allows Him to be a just judge – he has no axe to grind and no self-serving agenda.
Furthermore, He knows all things about a person – including their motives.
Now you can see why the Bible teaches us that “All have fallen short of the glory of God? (Romans 3:23).
If God were to judge us based on our own merits and our own works, we would all be screwed anyway.  For this reason, He made a way to be redeemed from our rotten situation – infected with the stuff the Bible calls “sin.”
He certasinly and obviously knows full well that not all people will be presented with this message in such a way that they can understand it – He is not that easily surprised!
However, He makes in clear that He is just – it is a promise that we can count on. In fact, the fact that He sent His only Son is proof that He is just.  Let me show you how I understand it:
I. God created man because He was full of love and wanted to share it… as one group of scholars stated – “The chief end of man is to glorify God and enjoy Him forever.”
This is the only thing that makes sense to me, if He made us in His image, as the Bible claims.   There is a lot of debate over what this means… but I want to take a quick look at one of the many implications of God making us in Him image:
As a dog owner, you will recognize that one of the motivations for you to try to make your dog more in your image (to know your commands, recognize you, not actually look like you) is so that the two of you can relate and enjoy one another more.
Further, probably the reason you want to have a child is to love them and share yourself with them.
II. In order for us to love Him, He had to give us some free will – some power over our own lives and destiny.  Love MUST be a choice, or it is not love.
III. If we have free will, then we can also choose to reject Him through our decision to go against His best. We all do, and furthermore, The Bible teaches that our representative, Adam did; and he passes that failure (called sin) down to each of us. We are infected with it – it makes even our most pure attempts and motives tainted just a little with something rotten.
IV. We are divided from Him by our evil (or at least non good) decisions.
V. He and we have a problem. We are unable to be what we were made to be because of this separation. He cannot have the relationship He desires with us and still be Holy and righteous. He must judge our evil if He is to remain Holy and Just – He cannot say “ahhh forget it,? and remain Holy and Just. We need to somehow be made morally pure (Holy) for the relationship to happen. So…
VI. God requires holiness and righteousness from us, but we don’t have it to give. Who does have it?
Imagine God standing with His right hand out saying “Steve – I must have righteousness and Holiness from you in order to have the eternal relationship I desire and that you absolutely require (if you are to avoid judgment and punishment). Do you have it??
“Uh, no.? You say.
“Hmmm.”  God says and He makes a little nod. Then you notice He is holding out his left hand too. Something is in it. You see that it is a payment for your sin, and a covering for your unholiness – In this hand, He is offering holiness and righteousness – just what you need!
“You can have this” God says “My Son purchased it for you by living a perfectly moral life and then allowing Himself to be tortured and executed to pay for your sins and everyone else’s. It is free, since you can’t earn it anyway. Someone had to pay. He did.  Justice is met, holiness is offered.
This is basically the gospel message.
Jesus died to pay the debt of our sins, and even better rose to conquer death too so that we can live forever from today on.
Basically, accepting this free gift (see Ephesians 2:1-10) is what it means to become a Christian. Do you see how this shows that He is just – and righteous – do you think He wanted Jesus to suffer and die? No. But someone had to pay or God could not be Just, and He MUST be just or we have no confidence in Him.
Eph 2:1-10
2:1 And you He made alive, who were dead in trespasses and sins, in which you once walked according to the course of this world, according to the prince of the power of the air, the spirit who now works in the sons of disobedience, among whom also we all once conducted ourselves in the lusts of our flesh, fulfilling the desires of the flesh and of the mind, and were by nature children of wrath, just as the others. But God, who is rich in mercy, because of His great love with which He loved us, even when we were dead in trespasses, made us alive together with Christ (by grace you have been saved), and raised us up together, and made us sit together in the heavenly places in Christ Jesus, 7 that in the ages to come He might show the exceeding riches of His grace in His kindness toward us in Christ Jesus. 8 For by grace you have been saved through faith, and that not of yourselves; it is the gift of God, 9 not of works, lest anyone should boast. 10 For we are His workmanship, created in Christ Jesus for good works, which God prepared beforehand that we should walk in them.

Here is how this applies. If God is this just and this merciful and loving, then I feel safe leaving the judgment of myself and others fully in His hands. He makes it clear to me that my job is to tell others about this “great news” (literal meaning for “gospel?) and to live freely within His standards, as appreciation for what He did for me. –All of this only comes after I have accepted this free gift of “salvation.” Which not only means safety from judgment and punishment, but also freedom from the dominance of sin and death in my life (Romans 6-8).
What about those who do not hear?  How they respond is between them and The Spirit of God.
We do not know exactly what it will look like, but we do have confidence that no matter how He judges, He will be just and righteous and loving and merciful all at the same time.
Summary
Three part answer:
1. God will reveal Himself to some degree to everyone.
2. God knows all and can judge perfectly.
3. God has revealed Himself to be merciful and just through His Word and through His actions already, so we can count on Him to be so in judgment as well.
Basically, to paraphrase CS Lewis – either God can judge rightly or He cannot. If He can, we can be confident in Him to do so. If He cannot, we are all in the same hopeless boat, no matter what we believe!  I have decided to have confidence in Him to handle judgement.
Chris Legg