The "KIRPAAN" is NOT a "KNIFE" per se....just as the "sacrament" given in a Catholic Church is NOT a piece of bread....and the "Wine" is not reallyty the "WINE" one drinks in a Bar for example..
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/sikh-youth/5297-kirpans-knifes.html
The Sikh religion has TWO KIRPANS...One for MIRI - TEMPORAL WORLD and the Other for PIRI - For SPIRITUAL WORLD.
The MIRI KIRPAAN is used to FIGHT INJUSTICE and for the UNDERDOG in the Battle field...The PIRI KIRPAAN is used to "BLESS" Sacred FOOD given out in the Presence of the GURU Granth Ji in a Gurdwara.
Reference:: Sikh Philosophy Network http://www.sikhphilosophy.net/showthread.php?t=5297
Guru Hargobind JI, Sikhism's SIXTH Nanak wore these TWO KIRPAANS..and He BUILT the AKAL TAKHAT..seat of Sikhism's TEMPORAL POWERS right In front of the Harmandir sahib Built by His Father and Grandfather GUru Ramdass Ji and Guru Arjun JI which houses the SPIRITUAL HOME of Sikhism.
Jarnail Singh gyani