Those who do not get the opportunity to know and accept Jesus Christ as their Saviour will be saved, if they live according to their conscience. They couldn't be punished for not believing in Jesus, since they had no chance to hear of him. It is true whoever calls upon the name of the Lord shall be saved (Rom. 10:13). But as Paul asks, "How then shall they call on him in whom they haven't believed? And how shall they believe in him of whom they have not heard?" (V. 14). The inference from Jn. 15:22 is that the Jews would not have any sin, if Jesus had not come and revealed, salvation is only through Him. He is the true way that leads to life. I am the way, the truth, and the life. No one comes to the Father except through me (Jn. 14:6). Elsewhere-Jesus says, "I am the Door. If anyone enters by me, he will be saved" (Jn. 10:9). Regarding the Salvation of Gentiles who possess no law, what Paul says is this when they do the things, contained in the law, by nature, they become a law to themselves. They are saved when they follow the law written in their heart, their conscience (Cf. Rom. 2:13-16). Because God desires all men to be saved. As there is only one God, there is only one mediator between God and man, the man Christ Jesus, who gave Himself a ransom for all (1 Tim. 2: 4-6). "We trust in the living God, who is the saviour of all men, especially of those who believe" (4:10).
In conclusion we may say, faith in Jesus Christ is the sure way of salvation, but as regards to those who haven't heard of Jesus, living according to their conscience would justify them. Any good conscience is the abode of Holy Spirit and whatever good comes, comes from Jesus. Though they are not hearers of faith, unknowingly they are doers of faith, and this is what precisely justifies them (Cf. Rom 2:13). But we Christians, who have received the full truth, have the responsibility to lead our brethren to Jesus Christ (Rom. 10: 14-15). One may cross a river by a thin trunk set across the shores of a river, or by a spacious concrete bridge built over the river. Those who have found out the sure and easy passage are bound to inform others about it, so that others also may cross the river safely. Thus all the sheep must be gathered to one fold (Cf. Jn. 10:16).