The act of associating partners with Allah, is called Shirk. The Quran calls people to accept the oneness of Allah and prohibits people from associating partners with Him. The Quran also states that shirk is a big sin and Almighty will never forgive associating partners with him.
“Surely Allah does not forgive that anything should be associated with him, and forgives what is besides that to whomsoever He pleases, and whoever associates anything with Allah, he devises indeed a great sin.” (Surah an-Nisā’ 4:48)
Tawhid with regard to the Essence of Allah (S.w.T.) denotes confessing that the Essence of the Lord of the worlds is one, and that He is Eternal. He is the Orignator of every action and phenomenon, whether perceptible through senses or not. There is no other cause except Him. Belief in multiple causes is Shirk.
"There are two types of shirk, major and minor. Paradise is for those who are free of both. If any dies while he is committing major shirk, (* without repenting even though he knows he is doing so), will be in the hellfire and won't be able to come out. Whosoever was free from the major shirk, while committing some minor shirk and has more good deeds than sins, will be admitted to paradise. Also, whosoever is free from major shirk, but has committed minor shirk by which he increased his sins to be heavier than his good deeds, will be admitted to the hellfire."
