The "God" or the "Lord of the Sikh" is uncompromisingly monotheistic, as symbolized by "Ik Onkar" (one all pervading spirit), a central tenet of Sikh philosophy. However Sikhs believe that God also prevails in everything. The fundamental belief of Sikhism is that God exists, indescribable yet knowable and perceivable to anyone who is prepared to dedicate the time and energy to become perceptive to their persona.
The Sikh gurus have described God in numerous ways in their hymns included in the Guru Granth Sahib, the holy scripture of Sikhism, but the oneness of the deity is consistently emphasized throughout. God is described in the Mool Mantar, the first passage in the Guru Granth Sahib, and the basic formula of the faith is:
ੴ ਸਤਿ ਨਾਮੁ ਕਰਤਾ ਪੁਰਖੁ ਨਿਰਭਉ ਨਿਰਵੈਰੁ ਅਕਾਲ ਮੂਰਤਿ ਅਜੂਨੀ ਸੈਭੰ ਗੁਰ ਪ੍ਰਸਾਦਿ ॥
ikk ōankār sat(i)-nām(u) karatā purakh(u) nirabha'u niravair(u) akāla mūrat(i) ajūnī saibhan(g) gur(a) prasād(i).
There is but one all pervading spirit, and it is called the truth, It exists in all creation , and it has no fear, It does not hate and , it is timeless, universal and self-existent!
Knowledge of the ultimate Reality is not a matter for reason; it comes by revelation of the ultimate reality through nadar (grace) and by anubhava (mystical experience). Says Guru Nanak, budhi pathi na paiai bahu chaturaiai bhai milai mani bhane which translates to "He is not accessible through intellect, or through mere scholarship or cleverness at argument; He is met, when He pleases, through devotion" (SGGS, 436).
The Guru Granth consistently refers to God as "He" and "Father". However, this is simply because the Granth is written in north Indian Indo-Aryan languages (mixture of Punjabi and dialects of Hindi) which have no neutral gender. Since the Granth says that the God is indescribable, the God has no gender according to Sikhism.
Guru Nanak prefixed the numeral one (ik) to it, making it Ik Oankar or Ekankar to stress God's oneness. God is named and known only through his Own immanent nature. The only name which can be said to truly fit God's transcendent state is Sat (Sanskrit Satnam, Truth), the changeless and timeless Reality. God is transcendent and all-pervasive at the same time. Transcendence and immanence are two aspects of the same single Supreme Reality. The Reality is immanent in the entire creation, but the creation as a whole fails to contain God fully. As says Guru Tegh Bahadur, Nanak IX, "He has himself spread out His Own “maya” (worldly illusion) which He oversees; many different forms He assumes in many colours, yet He stays independent of all" (SGGS, 537).