From the Heart makes the right point, imo. We must come to Christ as hopeless, miserable, poor, wretched sinners and ask for his mercy, repenting from (changing our mind) what we think we can do to save ourselves and accept God's free gift, as all our righteousness, dead works, or anything else we do falls short. But what of those who never heard of Christ? Well, Romans and many places answer this, one is held accountable for the light given them, namely the creation showing a creator, and our conscience, God's word written in our heart, knowledge of right and wrong. Now, if one realizes they have a creator, and they know they have done wrong, and ask for forgiveness, I believe they are forgiven. If one has never heard of Jesus of course they will not be judged for rejecting Him, because they never did. But, if a person hears the true gospel message, that Jesus died, was buried and rose again, His death paying for all the sins of all the world, every last one, and rejects this wonderful, horribly costly gift, they have no hope, they have no payment for their sins, and must pay for them themselves. Theologians answer the question of 'those who never heard' better and more in-depth than I, but, for those who have heard, yet reject, or add to, (actually taking away from)this 'so great salvation', I know no other way by which we can be saved. We are all sinners, the wages of which is death, Christ died thus paying for all our sins, and if we trust in that, our sins are forgiven and Christ imputes His righteousness unto us in place of our own. So we do not have to go about 'to establish our own righteousness'. Christ explained how, according to all the things written about Him in the scriptures, OT, He HAD to suffer and die, according to scriptures, to fulfil all the things pertaining to Him. It is so simple, its just John 3:16: (I like 17&18 too...)
3:16 For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.
3:17 For God sent not his Son into the world to condemn the world; but that the world through him might be saved.
3:18 He that believeth on him is not condemned: but he that believeth not is condemned already, because he hath not believed in the name of the only begotten Son of God.
(King James Bible, John)