I was wondering if the American Indians in America prior to 1492 were saved despite their isolation from the rest of the world.
You might as well also ask about the people prior to Christ. The Israelites had God's law, but most of the world did not have the laws of God written down for them. The first three chapters of Romans deals with this issue.
Those without a written law were not righteous. They chose to forget God.
"For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who suppress the truth in unrighteousness, because what may be known of God is manifest in them, for God has shown it to them. 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, 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. Professing to be wise, they became fools, and changed the glory of the incorruptible God into an image made like corruptible man -- and birds and four-footed animals and creeping things" (Romans 1:18-23).
The American Indians were no different from the rest of the world in this regard. They worshiped various animal spirits in their religions. It is not that man never knew. Everyone descended from Noah and his family. You can even see this in Genesis. Several times the Gentile kings acted more righteously than Abraham and Isaac in their dealings. But something was lost over the years.
Paul argues that God has a right to hold people accountable for their sins, even when they don't have a written law. "For when Gentiles, who do not have the law, by nature do the things in the law, these, although not having the law, are a law to themselves, who show the work of the law written in their hearts, their conscience also bearing witness, and between themselves their thoughts accusing or else excusing them" (Romans 2:14-15). It is easy to show that people understand with no exposure to God's law that some things belong to a person and to take them without permission is wrong (stealing). Yet some people still do that, thus they are not able to reach the level of righteousness their own minds understand.
The Jews fared no better. They had God's Law spelled out for them, but they weren't able to keep it. "You who make your boast in the law, do you dishonor God through breaking the law? For "the name of God is blasphemed among the Gentiles because of you," as it is written" (Romans 2:23-24).
Thus, the point people keep overlooking: There were no people deserving of salvation in the whole world, "for all have sinned and fall short of the glory of God" (Romans 3:23). This is why God intervened to send His Son to die for us. "But God demonstrates His own love toward us, in that while we were still sinners, Christ died for us" (Romans 5:8). It is also why there is no other path to salvation, mankind certainly is unable to be righteous on its own. "Nor is there salvation in any other, for there is no other name under heaven given among men by which we must be saved" (Acts 4:12).
What about the person who never heard the Gospel? He could be judged by what he does know -- and he will come up miserably short. Nor does he have his own means by which to remove his shortcomings. I so deeply wish people would realize salvation doesn't come by accident. The tragedy is happening daily in our world because there is a cure for sin and people aren't spreading it -- the very people who ought to understand the urgency of getting the cure to people. "For we must all appear before the judgment seat of Christ, that each one may receive the things done in the body, according to what he has done, whether good or bad. Knowing, therefore, the terror of the Lord, we persuade men" (II Corinthians 5:10-11).