Max Arthur Macauliffe writes about the authenticity of the scriptures:
The Sikh religion differs as regards the authenticity of its dogmas from most other theological systems. Many of the great teachers the world has known, have not left a line of their own composition and we only know what they taught through tradition or second-hand information. If Pythagoras wrote of his tenets, his writings have not descended to us. We know the teachings of Socrates only through the writings of Plato and Xenophon. Buddha has left no written memorial of his teaching. Kungfu-tze, known to Europeans as Confucius, left no documents in which he detailed the principles of his moral and social system. The founder of Christianity did not reduce his doctrines to writing and for them we are obliged to trust to the gospels according to Matthew, Mark, Luke and John. The Arabian Prophet did not himself reduce to writing the chapters of the Quran. They were written or compiled by his adherents and followers. But the compositions of Sikh Gurus are preserved and we know at first hand what they taught.

Pearl Buck  a Nobel laureate, gives the following comment on receiving the First English translation of the Guru Granth Sahib:
I have studied the scriptures of the great religions, but I do not find elsewhere the same power of appeal to the heart and mind as I find here in these volumes. They are compact in spite of their length, and are a revelation of the vast reach of the human heart, varying from the most noble concept of God, to the recognition and indeed the insistence upon the practical needs of the human body. There is something strangely modern about these scriptures and this puzzles me until I learned that they are in fact comparatively modern, compiled as late as the 16th century, when explorers were beginning to discover that the globe upon which we all live is a single entity divided only by arbitrary lines of our own making. Perhaps this sense of unity is the source of power I find in these volumes. They speak to a person of any religion or of none. They speak for the human heart and the searching mind.