According to the Holy Quran, salvation cannot be monopolised by any single religion of the world. Even if new truths are revealed and new eras of light have dawned, those who live a life of ignorance through no fault of their own and those who generally try to lead a life of truth even if they inherited false ideologies, will not be denied salvation by God.
The following verses from the Holy Quran elaborate this point further:
[22:68] For every people We have appointed ways of worship which they observe; so let them not dispute with thee in the matter of the Islamic way of worship; and call thou the people to thy Lord, for, surely, thou art on the right guidance.
In another verse, the Holy Quran declares in the same context:
[5:70] Surely, those who have believed, and the Jews, and the Sabians, and the Christians —whoso believes in Allah and the Last Day and does good deeds, on them shall come no fear, nor shall they grieve.
As such, followers of all religions based on Divine revelation have been granted the assurance that provided they do not fail to recognise the truth of a new religion (despite their sincere efforts to understand) and stick honestly and truly to the values of their ancestral religion, they have nothing to fear from God and will not be denied salvation.
The Holy Quran, speaking of whichever party from among the Believers: Jews, Christians and Sabians, promises:
[2:63] Shall have their reward with their Lord, and no fear shall come upon them, nor shall they grieve.
And
[5:67] If they had observed the Torah and the Gospel and what has been now sent down to them from their Lord, they would surely, have eaten of good things from above them and from under their feet. Among them are people who are moderate but many of them are such that evil is what they do.
Reference:
https://www.alislam.org/library/books/IslamsResponseToContemporaryIssues.pdf