Significance - Writings of the Gurus & Sikh Saints
Some of the earliest known references to Sikh banners appear in the Guru Granth Sahib and a number of other important religious texts.
Written by Sikh poet Kal in praise of Guru Amar Das the third Sikh Guru (1479-1574), this semi-biographical account alludes to a white banner during the time of Guru Amar Das. 
Guru Amar Das founded the town of Goindwal and it experienced rapid growth as many Sikhs thronged there for spiritual guidance. Pilgrims moved there in large numbers to be close to the Guru and it is historically known as the first great center of Sikhism. Goindwal may have been the location of this early Nishan Sahib
The reference to the white Nishan Sahib being on top of a ‘bridge to heaven’ may have been a place of congregational prayers where Guru Amar Das would gather with devotees to impart his divine knowledge – providing a gateway/bridge to salvation/heaven.
Alternatively the ‘bridge to heaven’ may also refer to Baoli Sahib, the well with 84 steps that Guru Amar Das constructed. The Guru proclaimed that whoever recited the entire Japji Sahib prayer of Guru Nanak on each of the 84 steps with a pure heart before bathing in the Baoli would receive spiritual emancipation.
Additional References in Sri Guru Granth Sahib
There are a number of other references to banners which equate them to markers of truth, understanding, honour and righteousness.
Guru Angad, pg 150
karam pavai neesaan n chalai chalaaeiaa ||
By Your Grace, the banner of honor is obtained. It cannot be taken away or lost.
Guru Arjan, pg 193
dhulabh dhaeh hoee paravaan ||
This human body, so difficult to obtain, is redeemed
sathigur thae paaeiaa naam neesaan ||2||
when one receives the banner of the Naam from the True Guru. ||2||
Bhatt Mathuraa, Svaiyay Mehl 5, pg 1404
guroo samarathh gehi kareeaa dhhraav budhh sumath samhaaran ko ||
I have grasped hold of the All-powerful Guru; He has made my mind steady and stable, and embellished me with clear consciousness.
fun dhhra(n)m dhhujaa fehara(n)th sadhaa agh pu(n)j thara(n)g nivaaran ko ||
And, His Banner of Righteousness waves proudly forever, to defend against the waves of sin.
Bhatt Kal, Svaiyay Mehl 5, pg 1407
kul sodtee gur raamadhaas than dhharam dhhujaa arajun har bhagathaa ||6||
In the Sodhi family, is born Arjan, the son of Guru Raam Daas, the holder of the banner of Dharma and the devotee of God. ||6||
Other Religious Texts
The Nishan Sahib banner is mentioned in Benti Chaupai (Chaupai Sahib), a prayer composed by Gobind Singh. It is one of the five prayers that Sikhs are required to recite every morning; Benti Chaupai is also part of the Rehras Sahib, which is recited in the evenings.
Kharag ket mai saran(i) tihari. Ap hath dai lehu ubari
Khargg – sword, Ket – flag, Mai Sarn Tehari – To Seek Protection
I seek shelter of God, Who has the sword on his Banner. O God, give me Your own Hand and protect me.
The rest of the Dasam Granth primarily references banners in stories of Hindu Mythology and historical battle scenes.
Dasam Granth, pg. 583
In the centre of whose banner there is the sign of a lion, he is Ravana, the king of demons and has ill-will for Ram in his mind;
Dasam Granth, pg. 1166
Lakhs of lances and arrows will be discharged and the banners of all colours will wave in the war-arena; the superb warriors will fall on the enemies taking their shields etc. and in all the ten directions the sound of “kill, kill” will be heard.325.
Dasam Granth, pg. 1216
The religious banners fluttered everywhere and there seemed to be no thief or Thug anywhere; he had picked up and killed all the thieves and Thugs and had established one-canopy kingdom.170. The kingdom of king Raghu was such that the differentiation of a saint and a thief did not exist there and all were saints; his discus fluttered in all the four directions, which returned only on cutting the heads of the sinners.171.
Vars of Bhai Gurdas
Bhai Gurdas Singh II (not to be confused with the earlier poet scribe of the Guru Granth Sahib Bhai Gurdas) writes in Var 41, Pauri 19 in praise of Guru Gobind Singh:
jin sabh prithhavee ko jeeth kar neesaan jhulaaeae a
He triumphed over the whole world and unfurled the sacred flags.
According to Sikh scholar and poet Bhai Sahib Vir Singh in his annotation in the `Vaaran Bhai Gurdas Steek' Var 41 was likely composed around 1787.
1. Pashaura Singh, Khalsa Insignia & Nishan Sahib, www.esikhs.com/articles/khalsa_insignia.htm, 2003