Ideas of revolt and reform of decadent systems are always in the air, it may be for centuries, until some one man bolder than the rest stands out to give them free expression; and as John the Baptist preceded Jesus Christ, so Nanak was preceded by several reformers, whose writings are incorporated in the Granth itself.
But while Nanak had substituted holiness of life for vain ceremonial, Guru Govind Singh demanded in addition brave deeds and zealous devotion to the Sikh cause as proof of faith; and while he retained his predecessors' attitude towards the Hindu gods and worship he preached undying hatred to the persecutors of his religion.
Starting from the unity of God, Nanak and his successors rejected the idols and incarnations of the Hindus, and on the ground of the equality of all men rejected also the system of caste.
Little is known of the ministry of Angad except that he committed to writing much of what he had heard about Guru Nanak as well as some devotional observations of his own, which were afterwards incorporated in the Granth.
The legend of his choice is that Nanak with his followers was going on a journey, when they saw the dead body of a man lying by the wayside.
The peculiar conciliatory tendencies of Kabir were carried on with even greater zeal from the latter part of the 15th century by one of his followers, Nanak Shah, the promulgator of the creed of the Nanak Shahis or Sikhs - i.e.
He also was a Khatri, and was chosen by Guru Nanak in preference to his own sons.
Thenceforward the Sikhs believe the spirit of Nanak to have been incarnate in each succeeding guru.