There are five pillars of Islam and Tauheed is the very first pillar among five. Shahadah, Salah, Zakah, Sawm, Hajj. Shahadah is only the expression of Tauheed. Tauheed means that “We accept that there is no God except Allah” without any argument. The oneness of Allah (Tauheed) has described in Quran by Allah and Allah ordered us to believe in HIM (Allah). Prophet Muhammad (Pease be upon Him) also taught us the true meaning of Tauheed and He (Pease be upon Him) explained the importance of Tauheed daily during His (Pease be upon Him) life.
Islam taught us that there is only one Creator for all the creation and there is no mediating power which has a rule in it. So in The Quran, Allah says to the prophet Muhammad (Pease be upon Him): “Say: He is God, the One. God, the Absolute. He does not beget, nor is He begotten, and there is none like Him.” This verse is Surah Al-Ikhlas and it does not only elaborate the oneness of Allah but also teach us that what is the complete meaning hidden in the oneness. In the explanation of it, we can say that Allah is the one and only Creator and the sustainer of this whole universe (including human). He knows everything; He sees everything, even the things that a necked eye of human can’t see; He hears everything. There is no compare of Him and he is the one with all powers.
The true importance of Tauheed in Islam is like a seed is to tree. It is like the proverb “what you sow, so shall you reap”. The stronger the base you make, the stronger the belief you will have. A tree is the amazingly developed extension of a seed, so when you start with a true understanding of Tauheed, you will have a great belief in the end.
Shirk is to share someone with the oneness of Allah. When a human start praying someoneelse, he or she is denying the very first pillar of Islam and hence they get out from the circle of Islam. In Islamic teachings, Shirk is one of the greatest sins and Allah will never forgive the one who is sharing someone with the oneness of Allah. In The Holy Quran, Allah says: ““Allah (Subhana Watala) will not forgive association with Him, but He forgives anything less than that to whomever He wills. Whoever associates anything with Allah (Subhana Watala) has devised a monstrous sin.” [Al Quran, 4:48]