Nik,
I would explain it this way. Everyone has done something they regret. We have all spoken ill of someone, or talked back to our parents, or told a teensy little lie. God is so immenesly holy that He will not allow His temple (Heaven) to be desecrated by anything "unclean" (i.e. a sinner).
EVERYONE is a sinner. Only one sin ruins the chance of us getting into Heaven on our own. God knew this, so He gave us the Law to show us that we could not follow it to the letter. Then he showed us our way out of that condemnation: He came Himself as a sacrifice for our sin.
Even your hypothetical Hindu has done something wrong in the eyes of God. And God promised that everyone would in some way hear of the truth before death. So they are right that he had judged himself.
If you are curious about those that died before Jesus, this is what happened. God told His people his plan for them. There are many prophesies about the Messiah, who was to cleanse us from our sin. Those who trusted in God and had hope in the coming Messiah were also saved.
Hope this helps