This is because religions are derived from the messenger or prophets of God. God sent different prophets to different part of the world to enlighten people. Sikhs believe that all those that believe in One God, irrespective of their faith are devotees of God, whether he is a Sikh, Christian, Hindu, Buddhist, or a Muslim. Where does the name Hindu, Sikh, Muslim and Christian come from? It comes from the source that inspired you. If you happen to be in this part of the world and you were lucky enough to meet Jesus, then you became Christian but you are still a devotee to God. To Sikhs religion is temporary; there are no religions here after, there is just spirituality.