Guru Nanak's doctrine was strictly monotheistic. In a phrase reminiscent of the Upanishads, he called god simply ekh (one). He is eternal, infinite and all-pervasive. He is perennially self-existent and is the source of love and grace. God is one, but what he has created has reality. He is both nirguna and saguna, i.e., he is with attributes as well as without attributes, yet he is formless.
The affirmation of the reality, the ultimate ground of all that exists, was the central value in Guru Nanak's teachings. The main quest was for mukti or release. Truest virtue one can have is the loving devotion to God. By immersing oneself in nam, i.e. by constant remembrance of the Divine Name, one can attain moksha or mukti. This was freedom from ego and self bondage from the circuit of birth, death and rebirth. One could go beyond this contingent state, could transcend Sansara-the sphere of temporality by concentrating on god's name. According to him fasting and visiting holy places was not that important. The first step towards enlightment was the awakening that the experience of the supreme being was the only ultimate truth. This awakening must be accompanied by an intense love of god, self-surrender to him and complete faith in his will.
At the end of his Udasis, Guru Nanak settled with his family at Kartarpur. There a community of disciples grew around him. An institution of far-reaching importance was community refectory, where all sat together to share a common meal overruling distinction of caste and creed. A key element in this process of restructuring of religious and social life was the spirit of seva or self-giving service.
The society thus taking shape was the precursor to historical Sikhism. Its main inspiration was supplied by Guru Nanak. He determined its principal truths and doctrines. In his life-time, it had acquired certain institutional features-the sangat, i.e., Holy fellowship or community the Dharamasala and the langar. Guru's word bequeathed to it through his 'bani' or revealed utterance. It had its own script-'Guru Mukhi' - in which the bani was recorded and its own style of singing. Its constituents were all common people like farmers, artisans, traders. Caste, icon-worship and empty ritual were its common rejections. Its mainstay was fervent faith in the divine, truly ethical practice and a full acceptance of life. Its ideals of fraternity and Seva and its concern with day-to-day affairs were elements which defined the course of its future evolution.
Sikhism is opposed to the caste system and all Sikh men carry the last name "Singh", Idol worship is also rejected. These features show the influence of Islam. But the ideas of Karma and rebirth are accepted. In their religious life and ritual, Sikhs are very close to the Hindus.