Is God able to bring a person to a saving faith apart from a biblical witness (the written Word of God) and/or apart from a personal witness (another believer)?
Jesus answered that question most clearly when He said, “I am the way, the truth and the life, no man comes to the Father except by Me”. This includes all of the Old Testament Saints, (Hebrews 11). Romans 3:10-12, 14-18 states,
“There is no one righteous, not even one; there is no one who understands, no one who seeks God. All have turned away, they have together become worthless; there is no one who does good, not even one…. Their mouths are full of cursing and bitterness. Their feet are swift to shed blood; ruin and misery mark their ways, and the way of peace they do not know. There is no fear of God before their eyes.”
Without God’s grace there is clearly no salvation. Can we all agree on this point?
There are two people mentioned in the Bible that may support your view of a witness without a transmitted (from another believer), or written Word. One is Abram (Abraham) called by God from an idolatrous family. God chose Abraham from among idolaters. The other person that may support your theory is Melchizedek (Genesis 14). Melchizedek was king of Salem (the future Jerusalem), possibly a Canaanite. Melchizedek expressed a new name for God that Abraham had never before used, the God Most High (El Elon). Abraham recognized this and then used this new name to worship his God during the encounter. Abraham recognized that God’s special revelation was not limited to him. Melchizedek received Abraham’s tithes, received his blessing, and accepted his wisdom. Here was a man who was Abraham’s superior (Hebrews 5). God called Melchizedek from among the heathen. (On the other hand some scholars believe that Melchizedek was a theophany, an appearance of God).
On the other hand, God calls Abraham’s prodigy, the chosen people. There are no other people that He calls by this name. So the question remains, ‘Can God bring a person to a saving faith apart from a biblical witness (the written Word of God) and/or apart from a personal witness (another believer)?’ Yes, with God every thing is possible!
"The God who made the world and everything in it is the Lord of heaven and earth and does not live in temples built by hands. And he is not served by human hands, as if he needed anything, because he himself gives all men life and breath and everything else. From one man he made every nation of men, that they should inhabit the whole earth; and he determined the times set for them and the exact places where they should live. God did this so that men would seek him and perhaps reach out for him and find him, though he is not far from each one of us. `For in him we live and move and have our being.' As some of your own poets have said, `We are his offspring.' "Therefore since we are God's offspring, we should not think that the divine being is like gold or silver or stone--an image made by man's design and skill. In the past God overlooked such ignorance, but now he commands all people everywhere to repent. For he has set a day when he will judge the world with justice by the man he has appointed. He has given proof of this to all men by raising him from the dead." Acts 17:24-31
This brings us to a question regarding the election of God’s chosen. Does God choose us because we deserve to be chosen or because there is something special in me? Did God choose Abraham because Abraham had faith? No! God chose Abraham: No reason given! God chose Abraham.
“Abraham believed God, and it was credited to him as righteousness” (Romans 4:3). His people were called the chosen people.
Concerning Abraham and his people the Word of God says;
“For I have chosen him, so that he will direct his children and his household after him to keep the way of the LORD by doing what is right and just, so that the LORD will bring about for Abraham what he has promised him." Genesis 18:19
“For you are a people holy to the LORD your God. The LORD your God has chosen you out of all the peoples on the face of the earth to be his people, his treasured possession.” (Deut. 7:6)
“Hear this word the LORD has spoken against you, O people of Israel--against the whole family I brought up out of Egypt: You only have I chosen of all the families of the earth…" Amos 3:1-2
“But you, O Israel, my servant, Jacob, whom I have chosen, you descendants of Abraham my friend, I took you from the ends of the earth, from its farthest corners I called you. I said, `You are my servant'; I have chosen you and have not rejected you. “(Isaiah 41:8,9)
God chooses people to fulfill His perfect plan.
“Out of all the peoples on the face of the earth, the LORD has chosen you to be his treasured possession.” Deut 14:2
“All the nations gather together and the peoples assemble. Which of them foretold this and proclaimed to us the former things? Let them bring in their witnesses to prove they were right, so that others may hear and say, "It is true."
"You are my witnesses," declares the LORD, "and my servant whom I have chosen, so that you may know and believe me and understand that I am he. Before me no god was formed, nor will there be one after me. I, even I, am the LORD, and apart from me there is no savior. I have revealed and saved and proclaimed-- I, and not some foreign god among you. You are my witnesses," declares the LORD, "that I am God. Yes, and from ancient days I am he. No one can deliver out of my hand. When I act, who can reverse it?" This is what the LORD says-- your Redeemer, the Holy One of Israel: "For your sake I will send to Babylon and bring down as fugitives all the Babylonians, in the ships in which they took pride. I am the LORD, your Holy One, Israel's Creator, your King."
…the people I formed for myself that they may proclaim my praise. (Isaiah 43:9-15,21).
A good book to read about Gods working among the unevangelized is “Eternity in their Hearts, (the untold story of Christianity among folk religions of ancient people)” by Don Richardson. (Regal Books, 1981). Don Richardson has also written a great book called “Peace Child” about evangelizing the people of New Guinea.