What do we know of God?

Guru Nanak Sahib Ji describes the attributes of God in the Japji Sahib (the first prayer in Guru Granth Sahib Ji). In the Mool Mantar, the initial invocation to God in the Guru Granth Sahib Ji, it says:

ੴ ਸਤਿ ਨਾਮੁ ਕਰਤਾ ਪੁਰਖੁ ਨਿਰਭਉ ਨਿਰਵੈਰੁ ਅਕਾਲ ਮੂਰਤਿ ਅਜੂਨੀ ਸੈਭੰ ਗੁਰ ਪ੍ਰਸਾਦਿ ॥
“There is but one Universal Creator God. His Name is Truth and Everlasting. He is the Creator & Doer of all, pervading everywhere. Without Fear. Without Enmity or Hatred. Timeless Form. Unborn. Self-existing. Realised through the divine grace.”
(Ang 1)
Sikhi rejects the theory of incarnation. God does not take birth. He is self-existent and not subject to time; He is eternal; He can be realized through (His own grace or) the teachings of a spiritual guide or Guru, but such a guide must be perfect.
Sikhi believes in a personal God. The devotee is compared to a bride yearning for union with her husband and waiting on his pleasure to do his bidding.
The Gurus have called God by different names-Ram, Rahim, Allah, Pritam, Yar, Mahakal. There is no such thing as a God of the Hindus or a God of the Muslims. There is the “Only One Universal God” who is a presence, and is called Waheguru by the Sikhs (wonderful enlightener and wonderful Lord).
Is God transcendental or immanent? He is both. He is present in all things and yet they do not cover His limitless expanse. When God is seen through the universe, we think of him as Sargun (Visible/ Manifest); when we realize His transcendence, we think of Him as Nirgun (Abstract/ Unmanifest). Truly speaking, God is both in and above the universe. God is the Whole and the world a part of that Whole.

A complete knowledge of God is impossible. Guru Nanak Sahib Ji says:

ਏਵਡੁ ਊਚਾ ਹੋਵੈ ਕੋਇ ॥ ਤਿਸੁ ਊਚੇ ਕਉ ਜਾਣੈ ਸੋਇ ॥
ਜੇਵਡੁ ਆਪਿ ਜਾਣੈ ਆਪਿ ਆਪਿ ॥ ਨਾਨਕ ਨਦਰੀ ਕਰਮੀ ਦਾਤਿ ॥੨੪॥
“Only one as Great and as High as God can know His Lofty and Exalted State. Only He Himself is that Great. He Himself knows Himself. O Nanak, by His Glance of Grace, He bestows His Blessings. ||24||”
(Ang 5)

We can only have some glimpses of Him from His works. The universe is His sport in which He takes delight. The world is a play of the Infinite in the field of the finite.
By His order, all forms and creatures came into existence. It is the duty of man to study the laws of the universe and to realize the greatness and glory of the Supreme Being. He has created an infinite number of worlds and constellations. The world in which we live is a small atom as compared to other worlds. Scientists like James Jeans, Hoyle and Narlikar have confirmed this theory.