The spiritual source of the Khalsa, the order of those who put themselves directly under God’s command, is the Guru Granth Sahib. Guru Gobind Singh’s Khalsa does not need any justification from other sources.
Guru Nanak received God’s light, and that light was passed on to the subsequent nine Gurus. That light is also present in the Guru Granth Sahib, both in the verses written by our Gurus and in those written by bhagats, bhatts and gursikhs.
Guru Gobind Singh had to rewrite the Ádí Granth compiled by Guru Arjan, in order to insert the verses of ninth Guru. Otherwise he did not change a word, nor did he add any of his own writings. He also did not make any changes to the ‘nitnem section’ on the first 13 pages of the Guru Granth Sahib.
In 1708 Guru declared the Guru Granth to be the eternal Guru of the Sikhs, and the Guruship passed to the Guru Panth (those on Guru’s path), acting on the teachings of the Guru Granth.
Guru Gobind Singh did not change the Sikhs into professional soldiers in an army based on blind obedience. The behaviour of Guru’s Saint-Soldiers must be based on high moral principles, not fighting in anger and not taking what was not theirs.
The Saint-Soldier follows the concept of Simran and Seva. Simran means meditation, thinking of God, and this thinking of God leads to service to all in this case standing up against injustice and defending the oppressed.
Guru Gobind Singh loved the Guru Granth Sahib and his life was guided by its teachings. The Guru Granth Sahib is a flexible Teacher. There are no detailed instructions on how to live your life, which easily get outdated by changes in society. The Guru Granth Sahib teaches the underlying principles, simple values that are of all times and of all ethical systems.
Many Sikhs look for a justification of wearing the turban and the 5 Ks in the Guru Granth Sahib. These Sikhs make two mistakes 1) we do not need to justify the turban and the 5 Ks, the Guru asked us, and if we love Guru we obey 2) the Guru Granth Sahib, as I explained above, is the book of principles, not of detail. The verse that mentions ‘sábat súrat dastár sirá’ addresses Muslims, not Sikhs (P.1083).
If you want to offer your head, if you want to totally commit yourself to the Sikh way of life, start adopting the turban and the 5 Ks as a sign of that commitment and work towards becoming an Amritdhari Sikh.
It is possible to be a good person without wearing the 5 Ks, and wearing the 5 Ks does not make you a better person. But the combination of the commitment to a Gursikh jivan and wearing Guru’s uniform does make a difference as long as you see God’s presence in all, are the servant of all, and have humility.