Well that is why he calls us Christians to minister to the World, so those tribes people will go to heaven. Here is an interesting fact you might like.
A missionary woman i met was trying to reach a tribe in the africa that had not been in contact with the rest of the world for as long as anyone had records; a very long time. When she and the others went into the village they began work immediately. after many years she was able to communicate with them. What she found out was that the tribes people started talking about the beginning of the world. Curious she kept asking them questions about it. They revealed to her that they knew all about the , adam and eve, the fall of man, the tower of babble, the flood, etc. The tribe only had one God, it was the Lord. They didn't need missionaries is my point, they already beleived and had faith in him.
Maybe not all tribes have continued to beleive in God, but God does not completely give up on them. Here are ways God reveals himself to evryone.
1.Nature
2.Conscience
3.Vision's
4. Incarnate Word(Christ)
5. The written word (Bible)
6. Spoken word (God's Creation)
7. alot of others