This is one of those questions that everyone has had but the Bible does not address it clearly. The only place I can think of where the Bible dies seem to answer it is here, where Paul is arguing the importance of spreading the gospel to the Gentiles.
for, “Everyone who calls on the name of the Lord will be saved.”
14 How, then, can they call on the one they have not believed in? And how can they believe in the one of whom they have not heard? And how can they hear without someone preaching to them? 15 And how can anyone preach unless they are sent? As it is written: “How beautiful are the feet of those who bring good news!” (Romans 10:13-15, NIV)
However Nature is another kind of scripture, only much weaker in light. Therefore although the ordinary means of salvation is only through the gospel preached, I think their is some merit to the idea that God could theoretically save a person by His Spirit through the testimony of nature. The argument might be weak, but I do imagine that I could have believed in Christ by nature.
For example, say you lived on an Island with only yourself and animals. You would eventually understand death, you would sense and eternal power by the sight of the sky, Ocean, etc. Therefore you would conclude the designer of the world you live in is angry and you might become depressed and afraid of dying and facing his wrath. This part is reasonable, what is more difficult to imagine is does nature manifest God's love and grace in order that you could believe in a vague concept of Messiah and be saved. Even the idea of life being given for life could create a subconscious sense of grace, for men and various animals live by eating the life of another. Every day the innocent die to give life to the guilty (though the food is not willing, like Christ was, yet still Nature seems to provide it).
Although nature does not indicate God's love as much as His power and his finitude, there are some signs. For example, when you lay down at night is this not death, yet you rise in the morning. At night you see little, yet the Sun will rise promising hope. The birds of the air find food as though someone cares for them. Jesus himself drew attention to the birds and the lilies proving the care of an infinite God. The fact that God is angry yet you still live implies the infinite power is kind. So yes even a vague gospel seems present in nature. Therefore an ignorant man living on an Island could possibly become convicted of his sin, fear death, and cling onto the goodness, mercy and of God in nature. Yet in general this seems unlikely and certainly any such faith, if it existed, would immediately respond to the gospel if afterwards it was heard.
In conclusion I would not rule out faith in Christ through nature as possible but I have never met such a person. Actually I did read a book from a Missionary many years ago who met a tribe far removed from civilization that had a tradition that someone would bring them a message, and nearly the whole tribe became Christian as soon as the Missionary preached the gospel to them, but I forget the name of the missionary and the tribe.
