Such a Great Guru Sri Guru Nanak is, that instead of using my own words, I will present to you some of the words of Sri Swami Sivananda.
“Whenever there is a big catastrophe in the land, whenever there is decline of righteousness, whenever there are oppression and chaos in the land, whenever the faith of the people in God wanes, great men or saints appear, from time to time, to enrich sacred literature, to protect Dharma, to destroy unrighteousness and reawaken the love of God in the minds of the people. India was in a bad plight. Babar invaded India. His armies assaulted and sacked several cities. The ascetic captives were forced to do rigorous work. There was wholesale massacre everywhere. The kings were bloodthirsty, cruel and tyrannical. There was no real religion. There was religious persecution. The real spirit of religion was crushed by ritualism. The hearts of the people were filled with falsehood, cunningness, selfishness and greed. At such a time Guru Nanak came to the world with a message of peace, unity, love and devotion to God. He came at a time when there was fight between the Hindus and the Mohammedans—when real religion was replaced by mere rituals and forms. He came to preach the gospel of peace, brotherhood or the unity of humanity, love and sacrifice.”
“Nanak practised rigorous meditation in order to realise God quickly. He was always in a deep meditative mood. He did not care for his body. The parents thought that Nanak was ailing seriously and so they sent for a physician. Nanak said to the doctor: “You have come to diagnose my ailment and prescribe medicine. You take my hand and feel the pulse. Poor ignorant doctor, you do not know that the pain is in my mind. O doctor! Go back to your house. I am under God-intoxication.” Nanak gave up food and drink for some days. He became wholly absorbed in divine contemplation. He observed perfect silence. He concealed himself in the forests for days together.”
“Guru Nanak felt that it would be improper to postpone Nama Smarana or remembering the Name of the Lord, even by a single breath, because no one could tell whether the breath that had gone in would come out or not. Guru Nanak was a reformer. Guru Nanak attaches very great importance to prayer. He says that nothing can be achieved by man without divine favour. He says: “Approach God with perfect humility. Throw yourself on His mercy. Give up pride, show and egoism. Beg for His kindness and favour. Do not think of your own merits, abilities, faculties and capacities. Be prepared to die in the pursuit of His love and union with Him.”
“Guru Nanak invented the Gurumukhi characters by simplifying the Sanskrit characters. The holy Granth of the Sikhs is in Gurumukhi. It is worshipped by the Sikhs and the Sindhis. Every Gurudwara has a Granth Sahib. The beautiful composition of mystic poems uttered by Nanak is contained in ‘Japji’. It is sung by every Sikh at daybreak. The ‘Sohila’ contains the evening prayers. May the blessings of Guru Nanak be upon you all!”.