Ishvara or God: Revelation of God

                                                                  Q. Bhagavad Gita says that if a living entity does not know that Krishna is the Supreme Personality of Godhead, his life has no meaning. What I wish to know is what about persons who are from different religions like Muslims, Sikhs or Christians who do not know Krishna as the Supreme, though they are doing their duties faithfully. Are they differently treated by God? How can they attain the supreme planet of Lord Krishna?