1) Verily, Allah has created the world and everything in it in order to worship Him alone. He sent the Messenger to call the people to tawhid and the Noble Qur'an is concerned with the 'aqidah of tawhid in most of its chapters, and it explains the harm of shirk for the individual and the society. Shirk is the reason for destruction in this life and the reason for eternal destruction in the Fire of the Hereafter.
2) Verily, all the Messengers began their call with tawhid, which they were commanded by Allah to convey it to the people. He, the Most High, said:
"We have not sent before you from the Messengers, except We inspired to him that He is the only One worthy of worship, so worship Him."
The Messenger of Allah stayed in Makkah for thirteen years while he called his people to the tawhid of Allah and he called to Him alone without associating anything with Him. It was regarding this issue that Allah revealed the following verse to His Noble Messenger:
"Say: Verily I call to my Lord and I do not ascribe any partners to Him at all."
The Noble Messenger brought up his followers upon tawhid from the time of childhood and he said to the son of his uncle, his cousin 'Abdullah ibn 'Abbas, "When you ask, then ask Allah, and when you seek help, then seek help from Allah." [Hasan Sahih, Tirmidhi]. This tawhid is in reality the religion of Islam, which is built upon it, and anything other than it is not accepted by Allah.
3) Verily the Messenger, peace be upon him, taught his Companions to begin their call to the people with tawhid, so he said to Mu'adh ibn Jabal, when the Prophet, upon whom be peace, sent him to Yemen, "So let the first thing you call them to, be the testification that none has the right to be worshipped except Allah." And in another narration, " ... that they worship Allah alone." [Agreed upon]
4) Tawhid is embodied in the testification that "None has the right to be worshipped except Allah, and Muhammad is the Messenger of Allah." Its meaning is that there is no one who is worthy of deserving worship except Allah, and there is no worship except that which the Messenger of Allah came with. It is this testification which allows a unbeliever to enter into Islam, since it is the key to Paradise, and the one who testifies to it and believes it enters into Paradise as long as he does not invalidate it by shirk or a word of disbelief.
5) The disbelieving Quraysh offered the Messenger of Allah, peace be upon him, kingship, wealth and wives and other than that from the pleasures of this life in exchange for him abandon the call to tawhid, and abandon the attacks against the idols. However, he did not desire that from them, rather lie continued his call and endured the onslaught along with his Companions, until the call to tawhid was victorious after thirteen years, after which Makkah was conquered and the idols were destroyed. The Messenger of Allah, upon whom be peace, recited:
"Truth has come and falsehood has vanished, surely falsehood is ever bound to vanish."
6) Tawhid is the preoccupation of a Muslim in his life, so he begins his life with tawhid and he ends his life with tawhid. His task in life is to establish tawhid and the call to tawhid, since tawhid unifies the believers and gathers them upon the statement of tawhid. So we ask Allah to make the statement of tawhid (la ilaha lillallah) our last words in this world and we ask Him to gather the Muslims upon the statement of tawhid.