Tawheed is the greatest pillar of the five pillars of Islam. One can not become a Muslim without singling out Allaah (in all three types), approving his worship to Allaah alone, and denying all false deities.
Tawheed is the most significant of all tasks, and it is the first of all duties. It comes before all deeds due to its great status, and it is the first thing that we shall call others to.
The Prophet (PBUH), said to Moaz when he sent him to Yemen: (You are heading to people of the book “Christians”, let it be that the first thing you call them to is to single out Allaah.)
All acts of worship are unaccepted without Tawheed; it is the condition of its acceptance & the worship is not called worship unless Tawheed is involved, just as the prayers are not called prayers unless Wudu’u (purification) is involved.
Therefore, Worship without Tawheed becomes Shirk (Polytheism), damages all the deeds, and causes the person to be of the dwellers of fire.
However, Tawheed is the reason for security and guidance in the Dunya (this worldly life) & in the Akhira (the hereafter). The proof of that from the Quran is:
اىَزَِِٝ آٍَُْ٘ا َٗىٌَْ َٝيْجِغُ٘ا إََِٝبٌَُّْٖ ثِظُيٌٍْ أُٗىَٰئِلَ ىٌَُُٖ اىْؤٍَُِْ ٌَُْٕٗ ٍُْٖزَذَُُٗ
It is those who believe (in the Oneness of Allah and worship none but Him Alone) and confuse not their belief with Zulm (wrong i.e. by worshipping others besides Allah), for them (only) there is security and they are the guided.
Ibn Kathir (The Quran interpreter) said: (The meaning of this verse is that those who devoted all acts of worship to Allaah alone “according to the Quran and Sunnah”, without associating any partners to him are the secured ones on the Day of Recompense, and are well guided in this life and in the hereafter. Who meets Allaah with pure Tawheed is offered pure security and pure guidance. Shirk is the greatest oppression, and Tawheed is the greatest justice.)
According to the Hadith, it is believed that Tawheed is the reason for entering Janna (Heaven), and it is a reason for being saved from hell-fire. The noble Prophet (PBUH) said: (Allaah has prohibited the fire from burning whoever says “La Ilaha Illa Allaah” which literally means “There is no god except Allah”, with devotion.)
