And this is the condemnation, that light is come into the world, and men loved darkness rather than light, because their deeds were evil. For every one that doeth evil hateth the light, neither cometh to the light, lest his deeds should be reproved. But he that doeth truth cometh to the light, that his deeds may be made manifest, that they are wrought in God. (John 3:19-21)
The concept that everyone can be saved as long as they follow the "light" they have received is both true and a heresy. Huh? How can that be? If this is used to justify salvation of nonChristians who have heard the Gospel, then it is a heresy. The only possible "out" in Scripture would be for a pagan (not a Jew before Christ) to follow Truth before he knew it.
(For not the hearers of the law are just before God, but the doers of the law shall be justified. For when the Gentiles, which have not the law, do by nature the things contained in the law, these, having not the law, are a law unto themselves: Which shew the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another;) (Romans 2:13-15)
After they have heard the Gospel, then "doing the right thing" in ignorance won't cut it anymore. Note that this does NOT deny Christ as the only Way to the Father, let us look at the definitive passage on that:
Jesus saith unto him, I am the way, the truth, and the life: no man cometh unto the Father, but by me. (John 14:6)
Very clear cut there, right? I think so.
He that hath my commandments, and keepeth them, he it is that loveth me: and he that loveth me shall be loved of my Father, and I will love him, and will manifest myself to him. (John 14:21)
Aha, this resolves it, because if there is some "man on an island" who has never heard the Gospel, but responds in accord with God's Will will be one that Christ loves and He "will manifest myself to him."
Everyone receives the Light of God (see Romans 1:20) in their life, but it isn't the basis for salvation. Only when we turn towards that light and follow it can we become His children. We won't be judged on how much light God gives us, but by how we turn towards His light we have and follow it. Remember that when someone tells you that all religion is simply different ways to worship God, all light in this sense will lead back to Jesus. If they are going somewhere else in their beliefs, then they are denying Christ and accepting universalism.