Guru Nanak Dev Ji (15th April 1469 – 7 May 1539) has given a beautiful summary of his teachings in one of his hymns as follows:—
Love the saints of every faith:
Put away thy pride.
Remember the essence of religion
Is humility and sympathy,
Not fine clothes,
Not the Yogi’s garb and ashes,
Not the blowing of the horns,
Not the shaven head,
Not long prayers,
Not recitations and torturings,
Not the ascetic way,
But a life of goodness and purity,
Amid the world’s temptations.
Nanak Ji is also wrote the mool (central) mantra in Sikh holy book Guru Granth Sahib Ji :
"Ek Onkaar Satnam Karta Purkh Nirbhau Nirvair, Akaal murat Ajuni Sabhang Gur Parsad"
"There is only one God, His Name is true, He is the Creator, He is without fear, He is without enmity, He is timeless and without form, He is beyond birth and death, the enlighted one , He can be known by the master's Grace".
The Granth Sahib