Often, theologians divide revelation into general and special, and mean by “general” what can be known of God through nature. By “special” they essentially mean biblical revelation or at least the explicit gospel. So the question comes down to “can a person turn to trust God through general revelation”?
I think that’s exactly the wrong question. Biblically, no one turns to Jesus unless God draws them through either general or special revelation (by these definitions). This is the beginning of the Calvinist idea no one comes to God on their own. In Calvin’s understanding, God chooses some to be saved and draws them irresistibly to Jesus (or himself). No one turns to God on their own. Quakers agree that no one turns to God on their own, but believe that God draws all people to his son (or himself) at some time or times in their lives. This draw is resistible; humans must choose to trust God as he draws them. If they believe and trust Him, they are saved by grace.
Abraham believed God, and it was credited to him as righteousness. That’s the biblical idea. So, instead of phrasing the question into whether or not general revelation can be sufficient in the absence of special revelation, we would say that general revelation is never enough for anything beyond making people “without excuse” (as Paul says in Romans 1) and that God reveals himself and one’s need for Him to each human being on one or more “days of visitation” in which he makes it clear that we are insufficient in ourselves and that he is calling us to trust Him for purpose and direction in our lives at some level of understanding appropriate for the context. Then, we submit to him or we do not. Salvation begins when we submit to Him and trust Him as He reveals Himself to us. The explicit gospel is powerful and intended by God to be used in drawing many to Himself, of course. It is his will that we know it and spread it. But like Abraham, we must respond with belief to a time when God makes himself known to us. That can take place in the absence of anything but a human in the world and God reaching out to that human. It often takes place in the spiritual realm when the gospel is presented through apparently human means.
Further, no one is saved on the strength of their own reasoning through the words of scripture or logical argument. Rather, God speaks to a person, perhaps while he or she is reading scripture (actually, very often while
they are reading scripture – at least where it is available!) It is God speaking that is both necessary and sufficient to produce a context in which a human being can say “yes” in belief and it be credited to that human being as righteousness (or right relationship with God), and that human being be “saved”. It is God that must be present, gracious, and active, and nothing less.
See John 6:44 (No one can come to me unless the Father who sent me draws them) in addition to the many instances of Abraham believed God and it was credited to him as righteousness. To be justified is to be in right relationship with God. That is the issue. Are we in right relationship to God? God initiates the relationship. We hear him, change our thinking (repent) to trust and to agree with what he says, or we do not. God calls us to trust, enables us to trust, and empowers us to believe and change without overpowering our will. Once we have moved in trust, he calls us to move some more – again and again, toward maturity, for the rest of our natural lives. It is God’s plan that this takes place in and through serving Him and representing Him in Christian community. We are a part of a body – not the body by ourselves.