Will non-Muslims who die knowing nothing about Islam enter Hell? Are Muslims sinful for not calling them to Islam? Who is principally responsible for this?

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.

Muslims must call to the religion of Allaah The Almighty and guide them to the right path. Allaah Says (what means): {And who is better in speech than one who invites to Allaah and does righteousness and says, "Indeed, I am of the Muslims."}[Quran 41:33]   A group of Muslims must shoulder this responsibility without the least degree of laziness.

Once a person receives Da'wah but does not believe in it, he will be one of the people of Hell. The Prophet, sallallaahu ‘alayhi wa sallam, swore by Allaah that whoever hears about him whether a Jew or a Christian, but does not believe in him, will enter Hell. [Muslim]